A bruise is indicative of trauma.  So, some kind of trauma has to happen which allows blood to escape from a blood vessel into what's called the extravascular space.  So, it spills out and splurges out around the subcutaneous tissue in the body.  It can also happen in other tissues.  You can get bruising of your internal organs, contusions of organs, including the brain, for example.  But once blood leaves a blood vessel, the first thing that happens is that it starts to lose the oxygen that's locked up in the haemoglobin.  And when haemoglobin is full of oxygen, it's a nice red color.  So, that's why the injury first looks red. 

As the haemoglobin gets deoxygenated, it goes a bluey hue and then purplish.  And that's the darkening that's happening.  The next step is that the inflammation and damage also attracts the immune system.  So, white blood cells come in, trigger some more inflammation and they begin to eat up, engulf the red blood cells.  And when they do that, they break open the cells.  And out of the cells comes the haemoglobin, the red pigment.  And the protein part of that gets chewed up inside the cell, turned into amino acids and they get recycled or metabolized.  And this leaves just the central core of the haemoglobin called the haem group, which is what's known as a porphyrin ring.  It's a series of chemical carbon rings which have got locked away in their centre, an iron atom.  That's why it's red because of the iron. 

Now, what happens is an enzyme then kicks in called microsomal hemoxygenase, and this breaks open that ring, and it releases the iron which then goes off into the tissue and gets locked up with something called haemosiderin.  And that looks an orange colour.  So there's the orange colour in your bruise.  But the ring of hemoglobin, the haem group, initially gets turned in something called biliverdin, which is a green color, so there's the green in your bruise.  And that very quickly gets reduced to bilirubin, the yellow stuff.  And then that's why you also see yellow in your bruise.  And as the bruise ages, it's also affected by gravity because these materials begin to spread out in the tissue doing a, sort of, human chromatography experiment.  They spread out and different molecules get stuck to different things and travel at different rates.  And so, the bruise spreads out and you  see the separation of colours and the evolution of the colours.  But, eventually, all those substances just soak away and are taken away by cells in the bloodstream, and that's why the bruise eventually fades away and then you get better.  So there's your answer.

Diana -  Well, actually, as I mentioned in the news story earlier, the Sediba skeletons were dated using paleomagnetism and uranium-lead, which is an isotope.  With paleomagnetism, certain rocks they will have magnetic polarity which tells you what the magnetic polarity of the Earth was at the time that they were exuded from the crust.  And this changes over time.  So, you can work out when this rock was made.  With uranium isotope dating, there’s uranium-238 and uranium-235,which both decay to lead over time.  So, you can check the relative proportions of uranium and lead and work out how much has decayed into lead.  These isotopes have half-lives of a million to even 4.5 billion years, so you can actually go quite far back with your fossils. These are absolute dating methods, but there's also relative dating, which is the simplest method really.  You just look at the layers of the rock and say, "Well, this layer is below that one and we know that layer is so old.  So this must be even older."  And that's how a lot of fossils are dated.

Chris -   So, I guess what you're aiming to do is to use a number of different methods.  And it's a bit like drawing a series of lines to see where they all converge.  And you take the best agreement between all the different methods and say, well, that one seems to agree with everything and, therefore, it's likely to be that old.

Diana -   Yeah, that's right.  And the uranium-lead method is quite popular and it is actually quite accurate.  They can get down to 0.1% with the actual date, which is pretty good.

Dave -   The original way which doesn't give you an absolute date but does give you a relative date is that you look at the other fossils which are around it, because if you've got something like a dinosaur bone, around it there's also tiny things like beetles and little tiny snails. If you know these fossils only appeared at a certain date and went extinct another date, you know that the fossil must be from within those two dates.  You've got 40, 50 different fossils there, so you can get a really quite accurate date.

Chris -   Yeah.  As long as you get the other ones right, you're right.

The answer is because there is a very close relationship between the tooth and the gum.  They talk to each other.  The gums talk to the teeth, the teeth talk to the gums and also to the bones, and there's a big interplay between all of those tissues, which has what's called a ‘trophic’ or growth promoting effect.  And so, if you loose teeth, you also loose bone and gums, and vice versa.  So, it's because one thing is there to protect and support the other.  And if you take one away, the other one realizes that its role is now a bit more redundant.  So, the gums thin.

Dave -   That sounds like a lovely idea.  The problem is, there's nothing actually around the Earth holding the atmosphere in.  There's not like a great big greenhouse holding the atmosphere in.  If you keep on going up, you just hit the vacuum of space anyway.  So what's holding the atmosphere down?  It's just gravity.  The Earth has got enough gravity to hold even the tiny molecule of oxygen down on it.  And the reason why there's so much air pressure pressuring on us now is all of the air above us is getting pulled down by gravity and it's pushing down on us about 10 tons per square meter.  So, if you put a big straw up into space, all you would do is have a straw with air in the bottom and there wouldn't be any air at the top.  I guess you could possibly pump the air out.  But you'd have to push it a long way up for it to get blown away.

Chris -   If you could make a straw 50 miles high, you presumably have got the technology to deal with it another way would be my argument, wouldn't it?

Dave -   And it would probably take more energy than not burning the fuel in first place.

Chris -   But an impressive sight, though, wouldn't it?

Dave -   It would.

Chris -  Well, I think there's two ways to look at this.  There's the simple, does a microwave as a form of light, a radiowave relative if you like, have any impact on the chemistry of food?  And then, the other aspect of this is, does it have any impact on the chemistry of the other things that you put on the microwave like the container that the food is in?  So, first of all, let's look at the food angle.  The reason people think microwaves are safe is that the energy in a microwave is too low to physically break the bonds that join atoms and molecules together.  It's called non-ionizing radiation.  And for that reason, we believe that microwaves are not unsafe.  They will not rearrange or mutate things in your food.  And they shouldn't, therefore, pose a threat.

But what microwaves could do is to interact with other things you put in the microwave like plastic containers.  And not all containers are necessarily safe to be heated up to the kinds of temperature that they might get to in a microwave.  Because one of the things that a microwave does have is hotspots and cold spots because of the way the waves work.  And that's why you have to have a turntable.  But that also means that when you put something in the microwave, you could end up with the plastic being in a hotspot and getting very, very warm and some chemicals that are added in making plastics—these include chemicals called plastisizers—can have unwanted effects.  And there are now big studies going on around the world to see if some of these chemicals are a health risk at the kinds of concentrations at which they are leeching out of plastic bottles.  And we did a show a couple of months ago on this subject, and we looked at various things.  There's one called Bisphenol A, which is in certain type of plastic, and also some polycarbonate bottles can also release these chemicals.  So, the bottom line is, at the moment, we don't know for sure.  There is evidence suggesting that plastics can produce chemicals.  It may be unsafe if you heat them up.  And, therefore, the best option and the safest option is probably to use a container which is safe in a microwave, either certified as safe, as such, or use something made of porcelain because that, we know, doesn't undergo any kind of changes.  Would you concur with that, Dave?

Dave -   Yeah.  The one other thing which affects food is if you've got these hotspots (I mean, it depends on your microwave and there are more modern microwaves which have less severe hotspots) is that you can overheat the food and destroy some of the goodness in it.  And so, although it hasn't been hot for very long, if it gets up to maybe 120 for a short period of time, it could destroy some of the vitamins and things.

Fundamentally, a battery is a chemical reaction going on, and part of that chemical reaction is it pushes electrons from one side of the battery to the other side.  So, actually, you have two halves of a chemical reaction, one which absorbs electrons and another that gives them out.  And the only way that the chemical reaction can carry on is when these electrons are going around and getting back to the other side through your circuit and they can do work when it's doing that.  Now, there are lots of different chemical reactions you can use and so, basically, the number of atoms – the number of molecules which can be active or which can move electrons across is important.  The more you can get in the battery, the more current, more charge you can store in the battery.

So, you've got to have lots of complex things that hold it together, which take up space and take up weight – and, normally, it's only on the surface where these things can react.  So, any kind of centre of an electrode which isn't able to react with things is useless and doesn't work very well.  So, basically, the way they last longer is by having more of the battery, which is active, you can do different chemical reactions which can store more energy, take up less space.  You can also use different chemical reactions if to produce different voltages.  So, a lithium ion battery is at 3V whereas a standard one and a half volt alkaline cell is 1.5V, and a rechargeable battery is 1.2 volts. 

Different batteries have different properties.  Alkaline cell will last for a very, very long time.  It doesn't lose its charge.  It could just sit back and it will last for several years.  It has a shelf life of several years.  Whereas a rechargeable battery will just discharge itself in maybe a month or so.  Also, a rechargeable battery can give out much more current.  So, if you've got a high current application, a rechargeable battery works a lot better.  So, yes, there are different circumstances where some are better than others.

There was a wonderful paper written by a lady called Jennifer Brodmann, who is a researcher at the University of Ulm, and she was on the Chinese island of Hainan looking at an orchid called Dendrobium sinense.  Now, this is a really interesting orchid because no one knew what pollinated it.  It makes these beautiful flowers.  It's a white flower with a red centre, but it's rewardless.  In other words, the flower doesn't give anybody anything if they come and visit it.  So she decided to do a stakeout and she watched this flower , 121 hours of footage to see what came by.  And 35 insects paid a visit of which the majority - over 30 - were a kind of hornet.  And she thought, "That's interesting."  At closer inspection, revealed that these hornets didn't come in and spend much time loitering there.  They flew in and pounced on the flower and then abruptly left.  But when they looked more closely, they saw that as the hornet was doing the pouncing, it was actually depositing a bit of pollen on the orchid, fertilizing it and also picking up some pollen to take to another flower.  So they thought, "There must be something which is attracting this hornet to this flower."  So they made extracts of all the chemicals that come out of the flower and they found one really interesting one.  It's eicosen-1-ol.  And this particular molecule is a pheromone made by bees.  And, in fact, it's an alarm pheromone that bees make when they want to tell other bees about something exciting going on.  And what they realized is that this hornet species eats bees and it feeds the bees to its young hornet larvae.  So what the orchid is doing is making itself smell like a bee to attract a hornet, to get itself fertilised.  And it's doing it by making the same chemicals that the bees would and, thereby, fooling the hornet, so a wonderful example of sexual kind of subversion going on.

The point is that the plant has evolved to have the same genetic pathway or the same synthetic pathway that can produce these chemicals because this is the way in which it gets itself pollinated, and very effectively too by the look of it.

If you want to read it, it was actually published in Current Biology, last year, Jennifer Brodmann, a wonderful bit of science.

The simple answer to that is, no, it wouldn't work.  The general way that siphons work in general is that, essentially, you've got air pressure pushing down on both ends of your pipe.  One end is higher than the other, then that can push – push it on both sides, push the water up to the top and then it carries and going.  If there wasn't any air pressure, then you could form a bubble at the top of the tube and that bubble would expand and expand until the two levels of water are same level as that their two tanks.  Okay, so that's the simple answer.  The somewhat more complicated answer is that water has something called cohesion.  The water molecules do stick together as related to the surface tension.  And, therefore, as long as you don't form any bubbles, water can actually be pulled and actually will stay stuck together even at a negative pressure, even at zero pressure, or even as you can pull it apart, which is the reason why trees can actually go higher than about 30 meters which is the air pressure which will push the water up, or 10 meters, which push the water up.  And so even with a very, very thin tube and until you get a bubble, it would work for a bit but not for very long.

Chris -    I'll start with the simple aspect to this, probably good for me, which is that the way scientists work out how many calories are in food is quietly literally by burning it.  You use something called a bomb calorimeter.  And you put the food inside a container, you use an electrical element to ignite the food, and that's because you can log exactly how much electricity you fed to the element. So you can work out how much energy it took to get the food ignited.  And you soak up all of the heat that comes out of the burning food by water, and you measure the temperature rise in the water.  We know how much energy you have to put in water to make the temperature rise by a certain amount, so we can work at how much energy comes out of the food.  So if you subtract how much energy you have to put in to light in the first place and how much came out at the end of the day that tells you the gross amount of energy you could get out of the food.  But it is a bit more complicated, isn't it, Dave?

Dave -   Yeah.  The problem is that you don't absorb all the energy which you get from burning a food.  So things like dietary fibre which you can't digest – it goes straight through you and you don't get energy out of it.  Some things take more energy to digest than others.  So, I think, actually when they work it out, they do it from a great big table of ingredients and they work it out.  I'm not exactly sure how they work out how much food is absorbed from each ingredient.

Diana -   Yeah.  I think when we had Susan Jebb on to answer the question about how many calories are actually absorbed from food, she said that a lot of it does end up in your urine and your poo.  And also that bacteria will end up digesting a lot of calories in your gut and that the amount of calories that bacteria will get through actually varies between people's guts because they have different species in there.

Dave -   I get in trouble with this all the time with my girlfriend because I'm pretty clumsy when doing my teeth as well.  What's going on here is to do with the reason why T-shirts go fairly transparent when they get wet.  Basically, the reason why you can see a white object is actually, if you look at it under a big microscope it's transparent crystals or lumps.  When light hits it, it gets bent because light goes slower in it.  It gets bounced off in all sorts of different directions because the surface is very rough.  If you cover it with water that smooths out the surface and so light can get through much better so it looks dark.  As soon as that water dries out – so you've actually cleaned up most of the toothpaste but not quite all of it, then you can see those little particles, light bounces off them and it looks white.

Chris -   A bit similar to why paint looks lighter when it dries.

Dave -   Exactly the same thing.

Chris -   It's not just spiders.  There are many examples of organisms in the natural world that make toxins that are really, really toxic.  As one researcher put it to me, it is a case of overkill, and they do it incredibly well.  And the reason is because these animals have evolved to use and exploit a whole range of different prey rather than just one.  And this means that because the prey animals are different, they therefore need to make a range of different venoms, which have action against the range of different prey.  If they just use one, it would be very easy for one prey to evolve away from being sensitive to that particular venom.  By using a range of different venoms and therefore targeting a range of different prey, there's no chance that it will start working.  And over time, they've just continually updated and adjusted their repertoires so they have this amazing example of all these different molecules, some of which will work really well against one species, others less so.  And when they sting us, some of them absolutely take us to the cleaners.  And it's not just spiders, there are many, many examples that is going all around nature.

Dave -   Well, I guess also, some prey will evolve to be more resistant to them so that doesn't work as well.  So they have to get stronger and stronger.

Chris -   Yes.  But also, it means that some of these genes do, at the same time, become redundant, some of these venoms.  And so they actually lose the ability to use them and therefore those genes stop working.  They're still hiding in the genome, but they just don't turn those genes into products anymore.  But yes, they're very interesting. 

Diana -   Well, as far as I'm aware, no, and I think it would be quite big news if they had.  But there are all sorts of historical examples, and there was one example in some art made by the Moche, which was a group of people living in coastal Peru in South America around the turn of the first millennium. But basically about 300 A.D., they made some pretty pictures of them.  But there are also some examples about – I think it was 965 AD, there were some European Siamese twins were recorded in history, but as I said, as far as I know, no skeletal evidence.

Chris -   Presumably, because it would have been so hard to give birth to a Siamese twin.

Diana -   Yeah.

Chris -   Probably many perished and so did many mothers, and it would have just been the end of story.

Diana -   That's the thing.  And, also, neonatal bones of these – bones of babies really don't survive very well at all.