Stripping Down your Questions

On this week's snow-bound Naked Scientists, we're taking on your science questions! We discover the caterpillar that tricks it's ant hosts into treating it like...
08 February 2009


Closeup of spiral orb web


On this weeks snow-bound Naked Scientists, we're taking on your science questions!& We discover the caterpillar that tricks it's ant hosts into treating it like royalty, find out why fish get lost in acidic seas and why the gravitational pull of tonnes of ice may lead to greater sea level rise than predicted.& Plus, we find out what happens to salt after it's spread on roads to avoid ice, what processes make the sea salty and how scientists weight the moon.& In Kitchen Science, it's 'on your marks' for a microwave race - will water boil before ice melts?

In this episode

Maculinea rebeli - Adult (female laying eggs), Borbona (Rieti), Italy, July 5, 2003

Caterpillars kings of the mimics

Scientists from Oxford and Turin have found that certain species of caterpillars have developed the ability to use sounds to fool ants into accepting and nurturing them.

Maculinea rebeliWriting in this week's Science Francesca Barbero and her colleagues describe how they used sensitive microphones to eavesdrop on Myrmica schencki ants, the nests of which are invaded by caterpillars of the rare blue butterfly Maculinea rebeli.  Somehow these caterpillars persuade the ants not just to accept them but also to treat them like royalty, even feeding their own young to the hungry butterfly larvae if food runs short.

Scientists have known for some time that chemicals come into play and that the caterpillars secrete odour molecules that make them smell like an ant, but that couldn't be the whole story because it wouldn't explain why the larvae receive such preferential treatment in the nest.  This prompted the Oxford and Italian team to wonder whether the caterpillars might also be resorting to a form of sonic subterfuge to elevate their status because the ants communicate with one another through rasping sounds produced by rubbing together the rough surfaces of parts of their abdomens.

Queen ants produce subtly different sounds to workers, and recordings from the caterpillars in the nests show that they are mimicking the noises of ant queens.

Myrmica schencki - Queen Ant
Maculinea rebeli - Caterpillar

"This is the first time anyone has been able to record like this, with insects in their native environment and that are not distressed.  This is why we picked this up," explains co-author Jeremy Thomas.  "This is a wonderful example of evolution.  This blue butterfly group has a long history of a co-existence with this ant species.  Over time the caterpillars have adapted to exploit the ants by mimicking their signals, allowing the caterpillar to penetrate the nest where the ants protect and feed it."

West Antarctica

Gravity may make sea level rise worse

Gravity might have a major role to play in future sea-level rise, scientists are predicting.

If the world is going to warm up then this is going to cause the sea level to rise.

As water warms up it expands so it takes up more space and as ice on the continents melts it dumps more water into the oceans. Many models predict the melting of the west antarctic ice sheet which is predicted to produce a 5m rise in sea level, but researchers from the university of Toronto have noticed that these models haven't taken something important into account: gravity.

Everything with mass attracts everything else with mass gravitationally. So the 22 million billion tonnes of ice in the Antarctic ice sheet are attracting the sea water around them.

This lifts the local sea level creating a bulge around the ice sheet. So when the ice sheet melts you not only get a redistribution of the water in the sheetbut around it too.

Also, moving thousands of cubic kilometres of water could cause the Earth's axis to move by as much as five hundred metres, which will move where the centrifugal bulge of the earth is acting, changing the shape of the Earth and the oceans above it.

The overall effect of all this is that the sea level aroung North America and the Southern Indian Ocean could rise by an extra 1.5m on top of the 5m already predicted. The sea around West Antarctica could only rise 4m instead of 5.

The coronary angiogram (LCA) of a man.

A new urine test for heart disease

This week researchers have reported a new way of diagnosing whether someone is likely to develop coronary artery disease or CAD by testing the chemicals present in their urine.

The coronary angiogram (LCA) of a man.CAD is a major cause of death around the world and currently the only way to diagnose it is to conduct an angiogram, which is expensive and invasive involving injecting dye into the blood that shows up on x-rays and allows doctors to see how badly blood vessels are getting lined by fatty deposits or plaque, that can restrict and eventually block the flow of blood to the heart causing a heart attack.

Now a team of researchers led by Karlheinze Peter from the Baker Heart Research Institute in Melbourne, Australia have identified 17 specific peptides (the molecules that make up proteins) in the urine of people who were diagnosed as having CAD using angiograms.

Their study published in the Journal of proteome research describes also describes how Peter and his team carried out a blind test on these peptides. By first screening the urine of a group of volunteers, then subsequently giving them angiograms it was shown that the urine test accurately picked out the people with CAD around 84% of the time.

They also found a direct link between these peptides and CAD, because they found these same chemicals in the fatty plaques that line the blood vessels of people with the condition, which is presumably why these peptides are finding their way via the blood stream into the urine.

It is still early days for a new CAD urine test to be rolled out, but these findings point the way towards a cheap, non-invasive test, that one day may hopefully help to detect the condition much earlier, giving patients a chance to make changes to their diet and lifestyle to help control the development of the disease.

Barrel sponge (Xestospongia testudinaria)

13:24 - Finding the Oldest Evidence of Life

Tell tale chemical evidence of sponges represents the earliest evidence of complex life - in rocks up to 750 million years old...

Finding the Oldest Evidence of Life
with Dr Gordon Love, University of California at Riverside

Chris - Researchers have found evidence of the earliest animal life on Earth.  It shows that complex life was actually flourishing here much earlier in the Earth's history than we previously thought.  Dr Gordon Love who's at the University of California at Riverside has covered the chemical fingerprints of sponges which are thought to be the first forms of animal life.  He found it in rocks that are up to 750-odd million years old.  Hello, Gordon.  Welcome to the Naked Scientists.  What have you actually done?

Barrel sponge (Xestospongia testudinaria)Gordon - What we did was we looked through rocks in a certain line and we have a continuous sequence of rocks and an opportunity for looking at the geobiology of this time.  We found a continuous 100 million year record of sponges, going from a time known as the Cryogenian which is the time associated with two immediate glacial episodes and extending right up into the early Cambrian period.

Chris - Where did you get these rocks from?  How did you find them?

Gordon - We were fortunate we were working with an oil company in Oman, Petroleum Development in Oman.  They're just currently in an area of the world where people are producing vast amounts of oil from rocks of this age.  Through our contacts in the oil company we had access to very pristine material from drill cores.  That became very important in having this stretch of time and also the fact that these were very thermally well-preserved rocks that we were dealing with.

Chris - If you're looking at the fossil record though, and this frustrated Charles Darwin, life appears to pop into existence about 540 million years ago because prior to then presumably there weren't enough animals with hard body parts that could be fossilised.  You've presumably run into the same problem.  How do you know you've got sponges going back 750 million years?

Gordon - These are one of a number of steroid structures that we detected.  The precursor to these molecules, the sterols are very specific to a class of sponge called demosponges.  Sponges make a wide range of really unusual natural products which really interest the medical field as well.  But in this case despite about four decades of research precursors have never cropped up.  These precursors have never cropped up in single-celled organisms.

Chris - So what you're saying is you've now been able to unlock from the rocks chemical finger prints, if you like; molecules made by those early sponges 750 million years ago that when the animals died were locked away in those rocks.  Although the animals, the vestiges of them, physically are gone their chemical legacy lives on and that is what you can detect.

Gordon - Yes, I think that's right.  This is an important factor that Darwin was puzzled about.  The lack of information is that if you look at smaller and smaller scale then there is a tangible record for animals significantly pre-dating the Cambrian explosion.

Chris - Just tell us a bit about the technique to get these molecules out of the rocks.  You get your rock sequence from the oil driller.  How do you then get out of the rocks the chemicals from the sponges?  How do you know which rocks they've come from and that they were actually made by the early sponges all that time ago?

Gordon - We've a couple of approaches.  The conventional way in organic geochemistry is to powder the rock after you've cleaned or removed the outer parts.  Do some sort of solvent extraction technique, concentrated up the steroid components in the fraction and analyse these by gas chromatography/mass spectrometry.  We found a really abundant amount of these steroids and in all the different rock formations that we looked at.  To confirm that we were actually, it wasn't just coming from migrated oils that reside in the younger part of the section, what we did was we isolated what is an organic polymer.  The bulk of sedimentary organic matter is actually insoluble and it can't migrate anywhere and they make up a large macromolecule.  We basically broke chemical bonds by using a technique where we heat up samples with high hydrogen gas pressure.  When we looked again at the steroid composition we saw that again these sterols were abundant products.  In that way we could more confidently equate the age of the markers with the age of the rocks with uranium-lead isotope dating.

Chris - So this shows you that those rocks of that age had complex animals like sponges living in them.  Why do you think that they're there?  What does this tell us about the origin of animal life around on Earth at that time?

Gordon - Yes, most of the rock is in a depositional environment that we're looking at is shallow, marine waters.  I think originally the simplest animals like sponges would have colonised the sea floor on a shallow continental shelf.  It could have been tens of millions of years later we thought they could pervade into deeper water environments.  I think it's telling us that at this time in terms of the environment there was finite levels of dissolved oxygen near the sea floor - at least on the shallowest water environments.

Chris - Does it fit with what we understand was happening elsewhere on earth at the time?  Usually when you see an explosion of some evolutionary process there's something going on in the climate or in the Earth's other processes.  Can you marry this observation with anything else going on at the same time that might explain what you're seeing?

Gordon - Yeah, now we've pushed this back, the first appearance, into the time frame of two vast glaciation events.  My feeling on this is I go to many rocks of age which are older than the first glacial event and I have never seen any convincing evidence for sponge biomarkers.  I think that these major glacial events radically altered ocean chemistry in the aftermath by shutting down a lot of obscure interaction.  It seems, although we're still trying to get a handle on how exactly the chemical composition changed it seems they opened up avenues for new niches, especially for new organisms which could filter feed on the sea floor.

Portuguese Man-O-War jellyfish (Physalia_physalis)

Why do Portuguese Man o War jellyfish beach themselves?

Helen - It's a great question. Portuguese man o' war, a type of jellyfish but actually they're lots of different individual animals that live together. They're not really considered as being a single organisms which is rather intriguing. They can't actually swim. Other jellyfish have a way of pulsing their tentacles and moving so they're basically at the mercy of wind and currents and what's going on around them. They have this lovely blue and pink tinged pneumatophore full of gas that floats them on the surface. That catches the wind if the wind is blowing in that direction. The really cunning thing about these guys is that, like humans, Portuguese man o' war can be left-handed or right-handed. About half of them in the population has this pneumatophore that's shaped towards the left and half that's shaped towards the right which means they will go in different directions if the wind blows in a certain direction. I think that's very cool indeed. You can imagine why that might be a good thing. It means that half the population will get swept up on a beach if the wind's going in a certain direction. Some of them won't. They won't all necessarily land up on a beach 'cos that's not great. They usually get trodden on and will desiccate very quickly and dry out. It's all very much at the mercy of the elements.

Chris - I saw them in Australia, some of them doing exactly as you said. They were getting blown up on the beach because they have this sail on the top which pushes them along.

Why don't conductor rails short out in the snow?

Dave - There's a couple of things. The first one is that pure water is quite a good insulator. My girlfriend's dad used to work in a lab with huge amounts of very high voltages and very pure water. If water's pure it doesn't collect electricity very well. You need some salts in it.

Chris - Why doesn't it?

Dave - Basically, if you conduct electricity in a liquid you want some free ions. You want some positively-charged things, negatively-charged things. If you've got some salt in there the salt splits up into negative chloride ions and positive sodium ions. The positive ones move to the negative end of the battery and vice-versa. You get a current flowing. That's one thing and I think you do lose quite a lot of the power through things shorting out. I think you get some gentle losses through there all the time. If there's anywhere where it does get a lot of power dumped: if you get a particularly conductive bit of snow or ice it's going to get a lot of power going through it. It's going to get hot and it's going to melt and move away from the rail. It immediately breaks that short-circuit. Chris - Is it worth bearing in mind that the voltage on things like the London Underground is a lot lower than when you have trains doing long distances when it's much more economically sensible to use high voltages?

Dave - The rails on the ground are about 600V and the overhead lines are about 25,000Vbecause of this effect. If you had a 25,000V on a rail you would get sparks going to the ground. You'd also kill a lot of people! It's not actually lethal unless you touch it. I think they probably do lose a lot of power through them but less than you'd expect where if you do get a short it's going to burn out. Chris - On a foggy day if you go near to a pylon of high-tension cables where higher volts are used you can hear them buzzing as the mist is settling on the insulators supporting the cables. You get the effect you're describing where the little bit of arcing vaporises the dampness there and that goes away and then some more settles. Dave - Yes, if it vapourises it's going to expand the air out.

Schoolchildren writing on a blackboard

Why does the sound of nails on a blackboard elicit a physical response?

Chris - It's making me cringe just thinking about it. This is quite a commonly asked question. The answer is psychologists and scientists don't know for sure. One very plausible theory is that when you run fingers down blackboards the frequencies you hear are very high-pitched and the high-pitched frequencies are very similar to the frequencies that animals produce when they're in distress. On argument is that we are genetically programmed and tuned-in to be sensitive to those frequencies. That may alert us that an animal of our own species, perhaps, is in distress. Perhaps it's being attacked or eaten or is in danger. Therefore, by galvanising your attention and waking up and paying attention to that noise you're therefore on high alert and you can make plans to run away or fight. That's probably the best explanation there is for that at the moment. Helen - I like the idea of experimentation on other animals to see which ones are sensitive to nails on blackboards. We can put them in rooms perhaps and see which ones jump the most when we scratch our nails.

How do scientists find the weight of the moon?

Dave - The way they do it now is basically that if you've got something that's orbiting the moon its weight is going to produce gravity. The stronger the gravity the farther you can orbit. As soon as you put a satellite orbiting the moon you can measure its mass quite accurately. By looking at how a satellite's orbit changes as it goes around the moon you can see tiny variances in mass from mountains and things. You can get very accurate gravitational maps of the Earth and find things like ore bodies. If you're got very dense rock somewhere then that's going to pull you down a bit. In the distant past you might have worked it out the size of the tides on the Earth because you know how far away the moon is and how string gravity is on the Earth. By working out how much of an effect the moon has on the water you might get some idea of how much mass must be in the moon. Chris - You also know that the tides used to be much bigger than they are today because the moon and the Earth used to be much closer together. There are fossilised tidemarks that geologists have uncovered that are metres in height. Because the moon has slowly migrated away from the Earth because it is moving away from us by about 3cm a year. You end up with progressively smaller and smaller tides. We're now down to the more reassuring several metres rather than the 100s of metres that they were getting at one time. Dave - That effect is because the tides are slowing down the Earth and also speeding up the moon. You can actually detect the slowing down of the earth because 200million years ago you could tell how many days were in a year. The corals grow a little line every day in a year. You can see the range in a year of maybe 400 days in a year about 200 million years ago.

How do tube worms move between different hydrothermal vents?

Helen - That's a great question. These wonderful two metre long creatures are giant tube worms that live in the middle of the ocean, very deep down in the deep dark ocean where there's no connection to the light and they only survive because they have this symbiotic bacteria that harness chemicals that live inside them. How do they move from one vent to another? They do live in very clustered environments a long way apart from each other. There've been a couple of different studies that look at the genetics and first of all they found out how long their larvae can live for. One theory is that they have eggs and sperm. We can see them fertilising externally, outside of the worm. They form larvae and in the laboratory those have lived for 38 days. The idea is that that's enough time for them to hitch a ride on a plume of water. We know there are these nutrient buoyant plumes of a mixture of hot and cold water very deep down, we're talking km down in the middle of the Pacific Ocean. That's enough time for them to drift and find another vent for them to live on. These are also very short-lived things. These hydrothermal vents come and go as changes in the sea floor take place. Really they're quite ephemeral and that's one thing that they've done. Genetics are likely to be what happens. You've got very distinct populations that are fed by just a few larvae arriving and starting a new population as new vents open up black smokers and things like that. We're talking 400 degrees centigrade. Crazy ecosystems.


34:49 - The Future of Home Electronics

Meera joins Chris Vallance and Mark Ward to discuss the Consumer Electronics Show, and what to expect in home electrical appliances in 2009...

The Future of Home Electronics
with Chris Vallance, Mark Ward

Meera - It's time to find out what the world of technology has to offer in 2009. This month I'm in the business centre for BBC news online. I'm here with our resident tech expert, Chris Vallance.

Chris V - Hi, there's a reason why we've come up here. That's so  I can pick the brains of the BBC's resident tech expert, Mark Ward who is the technology correspondent for the news website. We're going to chat about trends in technology. It's the start of the year. There's one event which really showcases the tech industry: CES - the Consumer Electronics Show and Mark, you were there. It was at the beginning of January that the big conference happened. What was being showcased?

3D TVMark - Lots of things, really. It's a consumer electronics show. The clue's in the title. Everything was there. The big surprise was that I was expecting lots of funky shiny stuff but the big thing there was TV. Bigger screens, obviously but more stuff to do with your big, flat panel TV you bought last year. It's be either adding web bits to it or doing more with your high-definition movies and things like that. TV was the star of the show.

Chris V - And it was a bit 1950s as well, 3D TV?

Mark - Yeah. This was very much the kind of next generation, stereoscopic so you still have to wear a better quality of 3D. The problem is that, for all the talk you're still going to have to buy a 3D capable set. That's going to be pricy and there's very little content out there at the moment. Broadcasters will when there's content and the content makers will say we'll make it when there's an audience. At the moment it's a lot of plans but precious little action.

Chris V - We have seen things like IMAX which have a similar problem. We have a huge cinema, you need special cameras. They have taken off so it's not hopeless for 3D TV.

Mark - No, there's a lot of commitment in the film industry to do it. In 5 or 10 years time we might start to see that percolate through but people keep their TVs for a long time. If you've just bought one you're not going to spend the same amount again just to get 100 movies. There's a vast catalogue of better stuff you could get at.

Chris V - I think for 2008 I've got in my hands one of these little tiny netbooks, these small very ultraportable laptops. They're not very expensive and they've been a real hit. Is that going to continue next year, at least according to the people at CES?

ASUS Eee PCMark - Yes, there's a lot of tiny, shiny computers on show. Netbooks now are about 50% of the PC market. Improved by 100% last year, probably going to do the same this year. I think at CES we saw the end of the compromises you're going to have to make when you buy one. Typically it means they're a bti slow. Graphics aren't great and batteries can be a bit problematic. Whereas at the show last year we saw battery lives, much more powerful processors and graphics were starting to improve as well. You might be able to play decent  games on it and certainly show some video.

Chris V - But times are hard. Is that affecting what was on display at CES?

Mark - Well definitely. There  was certainly some stats quoted about sales figures, things like that. In certain sectors you're starting to see a shift from the kind of maximum people can afford in terms of a flat panel TV to something they're happy to spend. You see a bulge around the 32-40 inch sets rather than bigger than that. Certainly a lot of people were saying the consumers might hold up pretty well, actually. People have tended to have got their flat panel TV, got their games console, got their smartphone and now they want to do something with it. Games, movies, those kind of web-based services will be pretty popular over the next year as people stay at home rather than go out and save money.

Meera - mark, other than what you saw at CES this year is there anything else you're looking forward to in particular?

Mark - There's a growing emphasison green computers, green electronics, that kind of thing. I think Groucho Marx said about Doris Day - he knew her before she was a virgin. I suspect that's the same with a lot of electronics companies. They discovered they're not as bad in the green statistics as they thought. They're touting that as something that people should pay attention to. I think people are pretty sceptical. I think it's starting to figure in people's perceptions beyond brand and price. People are going to start taking those green credentials quite seriously.

Meera - What about you, Chris? What do think's coming up this year?

Nikon D700 cameraChris V - I'm interested in, it's not necessarily brand-new for this year, I think we're going to see more of it and people employing this technology creatively. I'm interested in these SLR, single lens reflex, cameras that can shoot video. There are two on the market at the moment at very different price ranges. The films that they produce are really interesting because you can change lenses which you can't do with a cheap consumer camera. You can put wide angle lenses, zoom lenses, you can even do those tilt-shift lenses to make everybody look like little tiny people. The creative possibilities are really interesting. The quality of images, because of the different way they focus onto the sensor, that looks very different as well. It's more like film. There are drawbacks. People who've reviewed them have said they're hard to handle, hard to keep them steady. You need tripods, you need to know what you're doing. I think the creative possibilities are really interesting high-definition films produced on these things start to appear on the web. I'm just really interested. I want to see what people make with this stuff. It's quite exciting from a visual point of view.

Does salt, put on the roads to prevent iciness, get into the water table?

Dave - That's a good question. Yes, all the salt is going to go down to the water table eventually, it's going to dissolve in water and run down where water is. It's either going to run down the streams and down into rivers and out into the ocean where it's not going to make much difference. The stuff which goes into the water table will make some difference. Locally it could be an issue. We are expecting when you put the salt down it will have a significant effect on the degree of saltiness in the ditches down the side of the road. Overall it's not going to be very big an effect just because of how big the UK is. 1mm of rain over a square kilometre is the equivalent of a thousand tonnes of water falling on the UK. The UK's 40,000 square kilometres so just one millimetre of rain, less than a 20th of an inch is going to be equivalent to 40 million tonnes of water.

Chris - Although to be honest that's not all falling evenly and uniformly like that. There could be some areas that could end up with local salt build-up but it's trivial in the grand scheme of things. Dave - If it's in a big enough area there's going to be enough rain to dilute it down where it's not going to be a problem to drink.

Fiddler crab

What causes the water in the oceans to be salty?

Chris - This is quite simple and it comes down to the fact that we have a hydrological cycle. The sun puts energy into the Earth. Each square metre of the Earth's surface, on average, gets energy at the rate of about 1kW from the sun. This energy goes into the sea water, and the water molecules gain enough energy - sometimes - to evaporate. So you have water vapour, which leaves the sea and goes up to form clouds.

Those clouds then travel over to land. When they're forced to rise over things like high mountains then in order to rise they have to drop rain. They lose some mass in the form of water precipitation.

That fresh water comes down out of the clouds and lands on the ground. It goes into rivers and streams and picks up tiny amounts of minerals and salts, which it dissolves on its way, percolating through the ground. There's not that much there, so the water tastes fresh. You can just detect trace amounts of these chemicals in the river water and in pond water. 

As it makes its way down towards the sea it then takes with it those salts.

When the water then re-evaporates in the ocean, that's just fresh water evaporating. The salts get left behind. Over millions of years you then accumulate salts in the sea until you see the salinity that you see today.

The sea isn't actually going to get much saltier, because once you get to a certain threshold concentration you start to get other chemical reactions kicking in which limit the accumulation further of any more dissolved ions or salt. As a result it just contains at the level it's at.

Dave - Actually, where we get most of our salt from is the edges of the sea: very ancient shallow seas where there's lots of evaporation at the edge of a desert. The sea keeps flowing in, lots of water evaporates and the salt crystallises out. That often gets buried - there's huge amounts under the North Sea. There's quite a lot in Cheshire.

Chris - And you can go to Poland and some very famous salt mines which were these salt pans, weren't they? Absolutely amazing.

An electrically powered exoskeleton suit currently in development by Tsukuba University of Japan.

44:00 - Wearable robot or dancing suit?

Could you built a suit to help you do taiji or ballroom dancing, either by pushing you around, or just by stimulating your muscles?

Wearable robot or dancing suit?

We put this to Noel Sharkey, Professor of Robotics, University of Sheffield.

I don't think anybody has a plan to make a suit like that but I think that now your listener has said it someone might do it. My immediate thoughts are the exoskeleton suits that have been developed by the American military and also by the Japanese company called Cyberdyne. They're leasing these exoskeleton suits to elderly people at the moment. What it is, is you put the suit on your body. It's very thin, lightweight metal and it goes up your body so it will detect how your muscles are moving and then move as you want. It'll lift you out of your chair, you can run upstairs and you can lift heavy weights. I know they can be remote-controlled so you could have somebody remote controlling it and getting you to do the right dance steps or you could programme it to do the right dance steps. Another point I thought was that maybe you could use one of those sensitive dance floors. An ex student of mine has developed one at the University of Limerick, Niall Griffiths. What it is, is a floor covered with pressure sensors. Irish dancers can use this floor and it's made up of squares. The squares will light up to make the dancer know where to go. If they put their foot on it there's a pressure sensor that detects where they've gone.

What could happen in combining the suit and the sensor floor is the floor could light up and let the person know where to go. If they didn't go there immediately the suit could go there for them and give them feedback. That would be my solution.

Why does foil touching a tooth filling taste strange?

Chris - Did you ever do the thing when you were at school where you put your metal pencil sharpener in your mouth?

Helen - I've never tried!

Chris - Certain pens that have a clip that you put over your pocket and it's made of metal, a different metal to the pen. You suck those and occasionally you notice a tingly sensation in your mouth. Have you ever noticed that? Helen - I'm going to try now but my pen's plastic.

Chris - That won't work. The point is that when you mix two different metals together and you have an electrical conductor between them and you have an electrolyte - saliva has lots of salt in it so it's an excellent conductor. You can get a chemical reaction happening between the two metals. One metal, the more reactive on will form ions and it will give up electrons which will flow through the electrolytes to the less reactive metals. That's how ht e reaction occurs. As a result if you tough two metals, in this case you're touching with a filling - that's mercury-silver amalgam - the aluminium will dissolve to make some aluminium ions. It'll make some electrical current which you will experience in your mouth as this tingling sensation and it will also react with the saliva to produce some hydrogen and some oxygen gas perhaps on the two different surfaces. You will actually deposit some material on your filling. You're basically turning your mouth into a battery.


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