Do bubbles help washing up?

07 August 2011

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Do bubbles help or hinder when doing the dishes? Can we find evidence of material from Earth on the Moon? Can camera lenses cause fires? And is fluoride in drinking water safe? In this Question and Answer show, we tackle your science queries, finding out if higher air pressure means louder sounds and if plants from cuttings remain genetically identical over centuries. Plus, launching Lego men to Jupiter, making brain cells from skin cells, and how vampire bats home in on hot blood...

In this episode

01:46 - Earth's Multiple Moons

A paper published in this week's issue of the Journal Nature presents evidence that the Earth might once have had a second moon, based on a puzzling problem with the geology of the...

Earth's Multiple Moons

A paper published in this week's issue of the journal Nature presents evidence that the Earth might once have had a second moon, based on a puzzling problem with the geology of the Moon's surface.  Even to the naked eye, it's possible to differentiate the Moon's surface into dark flat volcanic planes, the seas, and much lighter coloured rougher mountainous regions, the highlands.  The visible shape of the boundary between these regions is what some have romantically called "The Man in the Moon".

What's puzzling is that on a body as large as the Moon, you'd expect the flat planes and the mountainous highlands to be randomly distributed over its surface.  In fact, the seas are tightly clustered together on one side of the Moon, whilst the other side is much rougher.  Such a large inhomogeneity seems beyond what could be expected from random processes alone.

The_moonIn this week's issue of Nature, this puzzle has led Martin Jutzi and his colleagues at the University of California reconsider the traditional view of how the Moon formed.  The standard view is that around 4.5-billion years ago, a planet around the size of Mars, sometimes called Theia, collided with the Earth.  Both planets were entirely melted by the energy of the impact, and a small globule of this molten rock separated from the rest to become the Moon.

Jutzi considers what would have happened if, instead of a single moon forming out of this collision, in fact two moons had been formed.  His conclusion is that two moons could have co-existed in orbit around the Earth for around a hundred million years, but they would eventually have collided.  Could such a collision have led to the lop-sided shape of the Moon that we see today?

One obvious problem with that idea is described rather well by Jutzi in his paper: when astronomical bodies collide, the result is usually to make holes in things -- craters and basins.  In this case, something made mountains on one side of the Moon.  So, Jutzi sets out to investigate whether a collision could ever deposit mountains onto the surface of a body.  And he argues that if the relative speed of the two bodies is small enough, there isn't enough energy to excavate a large crater, and instead the result is a large pile of rubble rather resembling mountains.

An interesting further outcome of Jutzi's computational models is that, in this scenario, the molten magma in the interior of the Moon is pushed towards the Moon's opposite hemisphere by the force of the impact, which might in turn lead to increased volcanism around the point on the Moon opposite to the point of impact, and so the formation of volcanic planes, as we in fact see.

07:07 - Lego men going to Jupiter

Nasa has launched 3 lego men on their way to Jupiter

Lego men going to Jupiter

NASA's most recent mission to Jupiter called Juno was launched this week with a crew of three. The three aren't normal astronauts however, they are  3 Lego figures attached onto the spacecraft.

The figures are representations of the roman god Jupiter, his wife Juno, and Gallileo who was the first person to discover moons around jupiter, and for that matter around any other planet.

 Jupiter is the largest planet in the solar system, and contains more mass than all the other planets combined, and so probably started forming before the other planets. Juno is going to study the atmosphere and magnetosphere of Jupiter to try and discover more about its composition and structure.

The figures should be going on a long journey of about 2.8 billion km, first on an orbit that sends them just outside Mars, then getting an additional kick from earth's gravity in 2013, and arriving at Jupiter in 2016, when Juno can start its mission.

Unfortunately for any Jovian children who may come across these lego men, they are made out of solid chunks of aluminium, and aren't articulated, so playing with them might be rather dull. This is probably because the plastics used to make normal lego-men haven't been approved for use on space missions and might emit substances which could fog up a lens, or interfere with an instrument, but the idea is to inspire children on earth by taking something they associate with on an amazing journey.

10:12 - Homing In With Heat - How Vampire Bats find Blood

How do blood thirsty vampire bats home in on the best place to bite and therefore guarantee achieving a trouble-free feed? Well the answer is that they've evolved their own built-in...

Homing In With Heat - How Vampire Bats find Blood
with Professor David Julius, University of California, San Francisco

Chris -   How do blood thirsty vampire bats home in on the best place to bite and therefore guarantee achieving a trouble-free feed?  Well the answer is that they've evolved their own built-in infrared detectors to pinpoint where the best blood vessels are and David Julius from the University of California San Francisco is behind the discovery.  Hello, David.

David -   Hello.

Chris -   What made you think that bats might actually be resorting to temperature to guide them to where they should sink their teeth in this way?

David -   So, it's been known for several decades that bats have these so-called pit organs on their face that are heavily innervated with nerve fibres that allow them to detect infrared radiation.  What we've done is to ask what the molecular underpinnings of that system might be.

Chris -   So how these special pit structures on their faces can actually pick up infrared or heat?

David -   That's right.

Chris -   How did you approach it?  What did you do?

David -   We're more generally interested in the whole mechanism of temperature sensation, how we, as humans for example detect things like hot and cold, and we've been interested in finding out how this works in animals that really take thermal sensation to the extreme in a way and use this in a different but generally related manner.  And what we did was use some new methods in genomics, what they call deep sequencing or DNA sequencing where we can profile all the genes that are expressed in different tissues, and we ask what kind of molecules are expressed in the nerve cells that send their projections to these heat-sensing pits, and are known to be involved in the infrared detection mechanism.  And we look through those to find molecules that might be involved in this form of what turns out to really be heat sensation.

Chris -   Okay, so we know that our skin is sensitive to heat and we have a pretty good idea how it detects heat.  There are various chemicals which are on the surface of nerve cells that sensitise those nerve cells when the temperature goes up.  So, are you saying that a variant of one of those is being used by the bats on their face in order to not just detect temperature but to specifically detect temperature relevant to body heat?

Vampire BatDavid -   Yes, that's exactly the case.  So, what we've shown is that the bat expresses a form of a protein that we also use to detect heat.  But the form of the protein that the bat expresses is in some ways optimised where the temperature required for activation is lower than it is for our heat sensors and enables them to detect body heat coming from their prey, from a blood supply from a cow or a pig, what have you.  So the basic underlying mechanism is the same as the one we use, but they have some little bit of genetic trickery that enables them to modify the protein so that it's more sensitive to heat and can pick up radiant heat from their blood supply.

Chris -   So they're using the same genetic machinery that they would use elsewhere in the body to pick up when they're being burned or things are getting too hot.  But in these special facial regions, they are tweaking the gene a bit so that it becomes more sensitive at a lower temperature so they can use those organs to see where there is heat radiating from the right bit of an animal they want to bite, so they can infer where the blood vessels must be.

David -   Right, exactly.  In a body of a mammal, the sensory nerve fibres for example that allow you to sense temperature, touch or pain are distributed into different - what we call ganglia - that contain clusters of nerve cells.  And those that innervate everything from the neck down are in one set of ganglia and those that innervate everything from the neck up are in another set.  And in our bodies those two sets of neurons are more or less the same.  There are some slight differences in the expression of genes, but pretty much what you see in one cluster of neurons is the same as in the others.  And in the vampire bat, what we found is that in this particular gene that expresses this heat sensor, the nerve clusters that send nerve fibres to everything from the neck, up to the facial area, which includes the heat-sensing pits, the expression of the heat sensors are different,  and the protein coming from gene is modified so that it takes on this different form.  And in fact, that's one of the big clues that tells us that this gene is likely involved in this specialised function of the vampire bat, namely infrared sensation, because it is modified and it's modified only in those clusters of nerve cells that send their nerve fibres to this region of the body that is involved in infrared detection.

Chris -   There are other animals that also home in on heat.  There are some snakes and vipers for example that aim for the hot spot because that's where they want to invenomate because they, I guess, figure that if they put the venom where the heat is, that's where the blood is so it will act most quickly, and then also guarantee a strike on the animal.  Do they use the same mechanism as your bats then?

David -   They use a mechanism that's related but in detail, different.  One of the great example of this in terms of pit vipers is the snake that lives out around my area here called the Western Diamond Back Rattlesnake, and it also has what we call "facial pits."  They're somewhat different in structure that a vampire bat but generally have a similar plan and they detect radiant heat say, from a squirrel or a mouse that they're trying to find in a dark burrow at night, so it allows them to see the animal as a radiant illuminated figure.  They use a protein molecule that detects temperature that's a member of the same protein or gene family as the one found in the vampire bat, and the one that we use for heat detection.  It's encoded by a different gene, but they're part of the same gene family.  And so overall, the mechanism is similar but the exact molecule that's used is different in its detail and in its structure.

Chris -   And just to finish up, David.  Given that you've got this new insight into how this gene can change its behaviour if you do what the vampire bats are doing to it, in other words, make it sensitive at a lower temperature.  How does this inform our understanding of how pain is signalled in the nervous system and could there therefore be some uses of what you've discovered?

David -   Yeah, so that's an excellent question, and a molecule that we express, which by the way I should say is the target for things like chilli peppers.  So, the molecule that we're talking about here that's involved in temperature sensation in the bats and in our own nervous system is what allows us to appreciate sort of that hot zing from chilli peppers.  We're interested in that molecule, as are many other labs, because there's evidence to suggest that its also modified by agents that are produced during inflammation and tissue injury that then sensitise the whole system so that you now for example would appreciate a lower temperature as being sometimes painful.  So the example would be, if you have a sun burn and then you get in the shower and the temperature is normally what you'd consider to be warm and very comfortable, you might consider that or perceive that as being noxiously or painfully hot and that has to do with the fact that these inflammatory agents are acting on this molecule to lower the threshold to heat and therefore, generate a perception of pain even in temperatures that normally you wouldn't consider painful.  And understanding how that occurs and how changes in these molecule and how the structure of this molecule is involved in those sensitisation mechanisms is very important for understanding pain hypersensitivity, especially in the context of tissue injury.  And looking at this structure of these special bat receptors gives us some clues about what parts of the molecule might be involved in those kinds of temperature shifts.

Chris -   Who would've thought it?!  We know that garlic drives vampires away but now, maybe the chilli attracts them.  David, thank you very much.  That's David Julius.  He's from UCSF and you can find the work that he was talking about published this week, it's in the journal Nature.

17:49 - Making brains from skin

A new way to turn skin cells directly into functioning brain cells has been announced by scientists in New York.

Making brains from skin

A way to turn skin cells directly into functioning brain cells has been announced by scientists in New York.

Asa Abeliovich, a researcher at Columbia University in New York, Nerve Celltook human skin fibroblast cells and, using a disabled virus, added a handful of regulatory genes called Ascl1, Brn2 and Zic1.  These normally act as forebrain transcriptional regulators, meaning that their role is to control the expression of other genes, but in this case they were also able to trigger up to 68% of the cells to begin to turn themselves into neurones.

And just like normal nerve cells, the newly-produced neurones were capable of firing action potentials (electrical impulses) and their electrical activity could also be altered with nerve-selective drugs or chemicals.  They also had thin axon-like processes capable of releasing nerve transmitter chemicals and, when injected into the developing nervous systems of mice, they survived and could even be found wired-up in the animals' brains after they were born.

In a formidable twist, skin cells from patients with Alzheimer's Disease were also transformed into brain cells that displayed the very same abnormal biochemical characteristics that typify the disease.  "This is the first biological inroad for the modelling of sporadic illnesses," says Abeliovich.

Although this technique is not the first to turn skin cells into nerve cells, previous attempts to do this have involved first turning the cells into stem cells, called iPS cells, and then triggering them to re-specialise into neurones.  But recent studies have shown that cells generated this way can be unreliable owing to mutations occuring in their DNA during the re-programming process.  However, this new approach, described in the journal Cell, is one way around the problem.

What sort of camera lens would be best for starting a fire?

Dave - I think the biggest effect is essentially just the aperture of the lens and how big the lens is to start with. It's basically to do with how much light you can focus in onto that spot of the sun, so you want essentially something which will work in the dark as much as possible - so as low an F-stop as possible. A wide angle lens will work better and get more heat in that small space and cause ignition. But the amount of energy you can get in is limited by the radius of your lens so a big lens is better.

The biggest effect I would say is you want to use a cheap lens because the focal length of these lenses - designed to focus onto a sheet of film at the back of your camera - that's probably only 2 or 3 centimetres away from the back of the lens. And so, any fire you start is going to be very, very close to your lovely camera lens, so use as cheap a lens as possible.

Is there evidence of Earth meteor impacts on the moon?

Dominic - You're absolutely right that most impacts of bodies onto a planet will give a lot of material escape velocity, and that material will be lost as debris to space. And so, there's a lot of debris out there in the solar system and the chances are that it will, within a few million years, collide with one of the planets. Now, if you've got bodies colliding with the Earth then statistically, that material is quite likely to end up on the moon simply because the moon is the closest body to us. It's only 400,000 kilometres away, about a hundred times closer than the next nearest object which would be Mars.

The problem would be actually identifying individual features on the Moon as being from meteor impacts from material coming from the Earth. If you just look at a crater, all you can say is that material of such and such a mass has impacted the Moon, you have to actually find the meteorite and recover it to start doing chemical analysis on that to work out where it come from. For example, if you can find trapped gases in there, you can try and match it against the atmosphere of one of the planets.

The Moon has no atmosphere and that means these meteorites will get quite a hard impact onto the surface and they will probably be totally vaporized in the impact - unlike, for example, a meteor impact on the Earth or Mars which is cushioned by the atmosphere and so, those objects can survive and be found.

Also of course, we haven't explored very much of the Moon's surface in comparison to Mars where we've had rovers roving the surface for the last 6 years or so, finding, say, dozens of meteorites on the surface of Mars. So we haven't really explored enough to find meteorites from the Moon even if they were there. So I think you'd be lucky to find them.

Do plants grown from cuttings share identical DNA?

Chris - Wow! So you have this rhubarb that you can trace all that way back. Isn't it amazing? Well, the answer is that plants obviously use DNA the same way that we do. Their cells contain a copy of their genome and that genome gets translated into the proteins that make all of the enzymes that make the biochemistry that keeps the plant alive and also, that creates its cells and so on, just like we do. But plants have a slightly lower metabolism than we do, so their DNA tends to accrue damage more slowly than ours does, so they don't tend to accrue mutations quite as quickly as we might, but that doesn't mean that they don't necessarily accrue mutations.

So when you're splitting your rhubarb, you're effectively cloning it because you're splitting an organism down the middle, and the beautiful thing about plants is that if you split them in two like that, you just get two offspring that are genetically identical because they came from one plant originally. If you then grow those up, they'll make a new plant and if you split that again, you'll get another clone of that plant, and those clones are genetically identical.

But there can be bits of those clones which can have DNA changes in them and a really good way of seeing this, [is to look at] variegated plants. Have you seen plants that have a white outer edge to a green leaf? If you look at plants like that, sometimes you'll find a stem coming off of it, where it seems to have lost the variegation. You'll see that the leaves are completely green. They've lost their white edging. Have you ever seen that?Suzy - Yes.Chris - What's actually happened there is that the genetic change which gave the plant its variegation - that white profile around the edge of the leaf - whatever that gene is, or little cluster of genes, that has changed or been mutated. It's 'reverted' as it's called back to the original stock which was the original genetic profile that gave the leaf a complete outline. There was no white edging to it. And so, the plant has a bud that gave rise to that stem produced a line which is genetically distinct. And the interesting thing is that's still connected to a plant which is variegated, but if you took a cutting from that bit of a plant, you would get a new genetic stock if you like, because it's slightly different genetically. So plants do acquire mutations. They do do it from time to time, but if you genetically sequence your rhubarb, it would be nearly identical with a few changes here and there, to the one that your grandfather was growing. And the evidence for this is that there are some plants which have propagated cloning like this for generations and generations, and generations, and they slowly evolve and adapt to the environment in which they find themselves, because there are pressures to them from the environment - pests and chemicals and nutrients and so on, but they don't change enormously.

29:30 - Understanding Rip Currents

Water sports – surfing, body boarding, kite surfing, etc have all become increasingly popular in recent years, but the number of people who are getting into trouble in the water has...

Understanding Rip Currents
with Dicken Berrimen, RNLI; Tim Scott, & Paul Russell, University of Plymouth

Chris -   Water sports - surfing, body boarding, kite surfing, and so on.  They've all become increasingly popular in recent years, but the number of people who are getting into trouble in the water has correspondingly also gone up and part of the problem are what are called rip currents.  These are unexpected and fast moving currents that can drag you out to sea even if you're a very good swimmer.  In fact, I had a little brush with one of these when I was in Australia recently.  Now though, a group of surfers and scientists, I suppose you could call them surfer scientists, are trying to understand the processes of these rip currents so that we can make beaches safer places to be.  Planet Earth podcast presenter Sue Nelson has donned her bikini and quite possibly a spotty handkerchief with knots tied in the corner, and she's been to Perranporth in North Cornwall to meet Dickon Berrimen from the RNLI, that's the Royal National Lifeboat Institute, along with Tim Scott, and former European surf champion, yes, he really is a surf champion, but he's also Professor Paul Russell from the University of Plymouth...

Paul -   It's a 3-year project and we're looking at the factors that cause rip currents to vary.  The factors are the incoming waves which drive the whole system, measuring the rip currents themselves using the drifters, and the shape of the beach - the sandbanks, the rip channels in the beach and how those change.  The crucial thing about the UK is that we have these large tidal ranges so the water is moving on and off the sandbanks quickly and the waves are changing quickly, and therefore, rip currents can turn on and off very quickly, and this is what causes a major hazard.

Sue -   Now you mentioned the drifters, part of the equipment that you're using to measure rip currents.  Tim Scott, you were involved in designing this equipment.  Take us through the equipment that you're using.  Probably best to start as it's already been mentioned with the drifter.  What's a drifter?

Tim -   2010 Mavericks surfing competition. The image was taken from a boat.The drifters themselves stand about a meter tall.  The base of them is made up of a cylinder which is about the size of a 2-litre drinks bottle and above that is a mast which extends up to about waist high.  The drifters themselves float, they're neutrally buoyant, so when we put them in the surf zone they float with the mast sticking upright and standing up.  They have a damping plate on the bottom which stops them from surfing on the waves, and they very effectively mimic what would happen to a person if they were trapped in the surf and they were moving around on the rip currents.

Sue -   How many would you use?

Tim -   Well we use between 15 and 25.  Each drifter itself has got a GPS unit that tracks its location and it takes a position every second.  We use a couple of interesting survey techniques where we can use a GPS base station to make these measurements much more accurate.  These enable us to get very accurate velocities and positions within the surf zone using these bits of equipment.

Sue -   Paul, we've got a sort of taste of some of the parameters that are being measured there.  What else are these drifters, these GPS tracking devices, actually looking for in the water?

Paul -   the drifters either get taken out to sea on a rip current or they get washed up back in by the wave, so we're continually reseeding them.  So what we're looking for is the changes in the system.  So not just measuring it once but continually changing it as the tide changes, as the waves change, and as the seabed topography, the sandbars change.  And we've just done a 6-week experiment here which encapsulated quite a range of conditions and we're coming back in October when the beach will be different and the waves will be different, and the rip currents will be different, and we'll be repeating all the measurements then.

Sue -   Now both Tim, yourself and Paul are keen surfers.  Have you noticed just purely with your surfing?  I mean, because there are people here, body boarding, taking surf boards out obviously to enjoy the waves.  Have you ever noticed yourself that there are certain times of the year or the day when the rip current seem to be stronger than usual?

Tim -   Yeah.  Well as a surfer, you spend a huge amount of time in the sea and you see rip currents all the time and you know, you're constantly in rip currents and moving around, and many surfers might not know exactly the physics behind what's happening but they're certainly observing them all the time, and they have a really good intuitive knowledge of how these rip currents work.  The fact is they do change all the time and from seconds, from groups of waves, all the way through to seasons and annually, you get different kind of rips on different kinds of beaches.

Sue -   Dicken, is this what makes them so dangerous as a lifeguard then as surfers like Tim have said that they're there all the time and they're really quick?

Dicken -   Yeah, potentially.  I mean, we got to remember that it's not the rip current that kills someone.  It's the inability to cope and potentially, the inability to see it in the first place.  So, this science backs up what we think we know already and that's also interesting to come out.  It gives us a sort of confidence in the safety message that we give.  But also the lifeguards will be generally risk assessing all the time, dynamically.  As the tides dropping out here at Perranporth, it will get worse as the tide drops, and certainly towards low tide is the most dangerous time that we know on this beach and that's built up from experience.  So, the guys will now be adjusting potentially where they put their swimming area.  They'll be adjusting the advice they give to swimmers, and they may possibly even go in the water themselves to prevent those accidents happening.

Chris -   Dicken Berrimen from the RNLI with Tim Scott, and Paul Russell from the University of Plymouth.  They were talking with Sue Nelson.

Do bubbles hinder cleaning dishes?

Dave - I think yes and no. You tend to get bubbles when you're using some kind of detergent. Detergents break down the surface tension of water which makes it a lot easier for it to dissolve things like fat - little molecules with oil loving tails and water loving heads. The detergent sticks to the fat, and dissolve it, and that it will still have the side effect of forming foams quite easily. Having a very stable foam though, which will sit on the surface of your washing bucket, isn't vital for actually doing the cleaning. That's just aesthetic mostly and if you're in a dishwasher or something, it actually really hinders it...Chris - Or washing machine.Dave - ...or washing machine. Actually, the washing machine is quite interesting. In the States, they don't make foam at all because they have top loading washing machines so you can't see inside. In Europe, we have side loading washing machines that we can see inside. They make a bit of foam, but having too much would stop the clothes moving around properly or block the jets of water in the dishwasher from hitting the dishes, and therefore they wouldn't clean as well. But with washing up water, I'm not entirely sure it's a bad thing because it will act to insulate the water, so if you're doing a lot of washing up and you've got a lot of foam on the top, it will actually keep it warmer for longer and heat is very, very important for cleaning. So I think it probably does help but not the way you think it does.Chris - Can you help me out on this one because my mum told me an old wife's tale which I think is true which is: if you've got biological washing powder which doesn't have an anti-foaming agent, it's not automatic washing powder, and you need to use it in a machine, you can put a cake of soap inside the machine and it stops it frothing up. And if indeed, you do have bubbles in the bath because I used to have a lot of bubble bath when I was a kid - if you put the soap in the bath, the bubbles will break down. So what's in the soap that actually doesn't agree with the bubble mixture?Dave - I'm not entirely sure. I'd have thought you're getting a reaction between the two detergent molecules and you're forming little micelles inside the water which are basically all the tails of one detergent molecule meeting up with the tails of the other one. You form little balls of detergent inside the water rather than sitting on the surface and making foam.

Does Earth's rotation affect flight times?

Dominic - Taking the first part of the question, the atmosphere is moving with the surface of the Earth below it, because there's friction between the surface of the Earth and the atmosphere. And so, as the atmosphere is moving with the Earth, when you fly up into it, you continue to move with the surface of the Earth.

The rotation of the Earth also creates weather systems, because the equator is moving very fast in order to get round a whole revolution every day, whereas areas close to the poles have to move less far. That difference in speed at different latitudes creates, for example, hurricanes and other weather systems. That leads to upwelling wind systems, which mean that, when you're flying across the Atlantic for example, it's much faster to go from the US to Britain than to go from Britain to the US.

Dave - You get stable high-speed winds high up, called the jet stream, which is moving with the Earth towards the East; this contributes significantly to reducing the flight times going from the States to the UK, because an aeroplane is travelling with the prevailing wind and therefore confronts less air resistance or drag.

Dominic - Going on to relativity, whenever you're moving at high speeds, time appears to run slow for you. That's called the time dilation principle and so, whichever direction you're moving in this plane, you're moving at high speed, and that will mean that the time would dilate slightly and you will age slightly less quickly. That won't depend from where you're going, that will depend upon the amount of time you spend in the air, and how fast you're going.

Is fluoride in drinking water harmful?

Chris - It's really interesting the fact that fluoride is almost universally used in many countries. It's done because there's an understanding of the chemical reason that fluoride strengthens tooth enamel.

Tooth enamel contains the chemical apatite which is a form of calcium phosphate. If you add fluoride to the diet, either in food, in salt, in drinking water, then you can add a fluoride atom to the forming calcium phosphate and you get flouro-apatite and this is much harder than just normal hydroxy-apatite - the normal stuff tooth enamel is made of. So you can actually strengthen your enamel significantly and that's the reason that it's done because it can give people very strong, very good quality teeth.

If you look at the levels of tooth decay that have happened since this actually was introduced, levels of tooth decay have plummeted in many places that fluoridate their water. In fact, studies have been done looking at this effect and it seems to suggest that the number needed to treat, in other words, the number of people who you have to get to drink fluoridated water in order to stop having cavities who otherwise would is about 6.

So, for every 6 people who drink fluoride-laden water, one person will not to get cavities who would otherwise have done. So that's amazing actually. It's a very big health impact.

But then there's a question: Are there any health disbenefits? And this is where it gets a bit murky because actually, there's very little good quality published evidence - one way or the other.

There was a very big meta analysis that was performed by the University of York, which is probably the gold standard and has been cited by agencies and organisations all over the world since. What they actually say is they have taken 26-plus studies that have looked at fluoride in water and looked at the health benefits - no cavities - and some of the disbenefits. So for instance, do people who have been drinking fluoridated water have more hip fractures for example? Because if it gets into teeth, it can also get into bones, and if it's in bones, it could potentially alter the strength and the integrity of the bone architecture. So it might make people more prone to fracture. There's no compelling convincing evidence that it does associate with more fractures.

They've done a similar thing for cancers and not found any association with cancers, but the University of York team say, actually, the level of evidence is quite poor and really, we do need some very big trials and some big studies in order to look at this properly because the evidence is really quite scant.

Dominic - Obviously, a lot of people use fluoride toothpaste or fluoride mouthwash, but what additional benefit does fluoride in the water give?

Chris - The tooth enamel is in a dynamic equilibrium. So, if you have acid in your mouth then you can erode some of the enamel. If you shift it towards an alkaline environment and there's calcium present, and some fluoride, you can rebuild some enamel. So what the toothpaste is aiming to do is to supply you with a ready source of alkaline environment - that's why there's bicarbonate in it - and some calcium, and some phosphate, and they put fluoride in there because then it gets into the matrix, it's being laid down as new tooth enamel in this dynamic equilibrium, and it strengthens it.

If you have too much fluoride then - this is one of the other things that the York study looked at - you can get something called flourosis. People who live in Essex especially (I was one of them), if you drink lots and lots of tap water and are exposed to fluoride, there's very high levels of fluoride in the water that you drink, and this can get impregnated into the teeth and it's more likely to cause tooth staining and you get a speckledy pattern on the teeth.

You'll never get cavities. They won't look that aesthetic but you never get any tooth decay and I've never had any fillings in my entire life. My teeth are really good actually and I put it down to the fact that partly I was from Essex, so there are some benefits of being in Essex apart from the fact that it's got some of the best schools in the country, it does have very good water as well, and I think cleaning your teeth very regularly with a toothpaste laden with fluoride is very, very important.

Does higher air pressure result in louder sound?

Dave - I don't know about deep sea divers, but the effect related to how efficiently you can couple vibrations - so get a vibration from the musical instrument, or your voice, into the air, then from the air into your ears. With all of these transitions, you tend to get some sound carrying on and some sound reflecting. And the more similar the material which you're going from and to, the more energy it gets transferred, so if you're going from one bit of air to another bit of air, pretty much all of the energy is transferred from one to the other. If you're going from air to a solid piece of steel, almost none of the energy is transferred and actually, vice versa; If you're going from a solid bit of steel almost none of the energy is transferred into the air and all of it is reflected. So if you have very, very high pressure, it increases the pressure of air, it's going to increase its density and make it seem to the sound wave more like the musical instrument or more like your ear so more energy will get transferred, everything will get more efficient, and I would've thought you would hear things louder.

How can one cable carry many signals?

Dave - There are a variety of different ways of doing this: In some of the older ones, you can basically mix your signal with a radio frequency and essentially send lots of different radio frequencies down the same wire, the same piece of copper, in the same way as you can send lots of different radio frequencies through space. Each one of those can have a different conversation on it, and you can send lots of phone calls down one piece of copper like that. You can also do something which is called 'time division multiplexing'. So that's whereby you send a hundredth of a second of one person's conversation, a hundredth of a second of another person's conversation, a hundredth of a second of another person's conversation... and you interleave them, and then you have some electronics which you can take those back out again.

Chris - That puts it all back together.

Dave - ...Back together again. Or more recently, in the last 20 years, you basically digitise everything and a phone signal can probably take maybe 10 kilobits of information. An optic fibre can take gigabits of information, so you can get thousands and thousands of phone calls down the same wire.

Footprints in sand on the beach

50:25 - Why would footprints in sand appear raised?

"I recently took this photograph of my foot prints in the sand and it was only after I noted that the "footprints" are not IN the sand they appear to be "raised" on...

Why would footprints in sand appear raised?

We put this to Dr Rob Jenkins, a Cognitive Psychologist at the University of Glasgow...

Rob - The questioner has sent in a wonderful photograph of footprints on a sandy beach. What's striking about the photograph is that the footprints seems to rise up from the surface of the sand rather than sinking in. In fact, the footprints are normal footprints. They're sunk in as you would expect. Their raised appearance is an illusion caused by the pattern of shadows. These shadows are ambiguous. They could result either from bumps lit from the top of the picture plane, or from indentations lit from the bottom of the picture plane. In the face of this ambiguity, the brain makes its best guess as to which is more likely and that is what we see. With this particular image, our brains make the wrong call. Why? Because they have a built-in bias to assume that light comes from above. This is a sensible rule of thumb because sunlight generally does come from above, but not in this photograph. Here, the sun is setting behind the photographer, below the bottom of the picture plane. This is evident from clumps of sand in the foreground that act as mini sundials. Under these lighting conditions, only indented footprints could create the pattern of shadows we see. So the footprints must be indentations after all.

Diana - So the problem is that our brains have a bias toward top down illumination which means that the brain tends to assume light is coming from above. This bias is so strong that it often competes and overcomes the clues our vision is giving us about relative depths. So when light comes from a slightly different angle, in the case of the footprints in low sun, our brain tries to tell us they're convex instead of concave.

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