It would depend on whether you took all the air out of the tube first, because if there was air resistance it would drop all the way down. It would be going quite fast at the bottom but probably not very fast because it would be losing lots of energy to friction. It would go over to the other side a bit, but then it would start falling in the other direction and it would start coming north again, and it would oscillate and eventually end up in the centre of the earth. If you took out all of the air from the tube, it would drop all the way through the earth and end up pretty much at the South pole, it would stop there and it would come back again. It would go backwards and forwards and would look like simple harmonic motion, a bit like a pendulum. It would take about 90 minutes to bounce back and forth. If you imagine that you don't take the air out, it's worth thinking about water as an analogy. Every time you go 10 metres underwater, you increase the pressure pushing down on you by 1 atmosphere. If you were to go all the way to the centre of the earth pretty soon the air would become so dense and there would be such a huge amount of atmosphere above you, it would be like trying to swim through a solid. Which is why the earth's core is solid; because of the huge pressure that there is down there. The thing is, you wouldn't be able to go very very fast because there would be this tremendous resistance. You'd actually fall probably forever, because you'd go so slowly.

Possibly because the top of it's exposed to the air, so you get more evaporation off the top. The bottom's going to fill with oil, the top's going to dry out, so the top's going to get smaller and it's going to curve upwards.

Now I've been having a look into this, and at Thomas Nagel's 1974 work, "What is it like to be a bat?". Basically, bats don't ever go the right way up. They fly sort of head down, and they hang feet up head down, and basically, that's the way they think the world is. There have been some experiments done trying to put bats in low gravity, and basically bats don't really care which way is up. There have been some experiments done with bats and gravity, where most things, if you put them in wrong gravity or reduced gravity, they start rolling over and they get very confused, whereas bats just don't really care. They can fly upside down, they don't really mind. So bats generally hang upside down rather cleverly, and they don't seem to mind which way is up. And here's a good bat joke for you: There's a bat hanging upside down in a cave, and he sees another bat standing on the floor and he says "What are you doing down there?" And the other bat says "Yoga". [Chris] I can beat that! There's three vampire bats in a cave. And they're really hungry. And they can't get hold of any blood to save their lives. And then one day one of them says "I'm so desperate I'm going out of this cave, I'm going to find us something to eat". So he goes flying out of the cave and five minutes later he's back. He's got his mouth drenched in blood. And the other bats go "where did you get that wonderful blood?" and he says "come to the mouth of the cave and I'll show you". And they all go to the mouth of the cave, and he points out and says "You see that tree over there?" and they say "yeah" and he says "I didn't".

The force of gravity is a different think to heat, light and sound. Heat, light and sound are vibrations of various different types. Sound is a physical vibration in the air, or in an object. Heat is another physical vibration, a very much faster one. And light is a vibration in space itself. So in order to insulate against them you've just got to get something to sort of dampen their vibrations. Gravity is a force. So it's actually something which pulls something. You can insulate against forces such as electromagnetism, because they've got positive and negative. And so if you want to, you can rearrange positives and negatives in order to insulate against it. But because you've only got one kind of gravity, you've only got positive gravity, you can't have the negative ones there which you'd need to insulate against it. So you can't, unfortunately, insulate against gravity. Though it might be very useful if you wanted to build a space ship or something. However, people do talk about anti-gravity though. I'm just wondering if you could really create anti-gravity maybe you could counteract gravity. So maybe that will be possible in the future.

It's not quite true that matter can't be destroyed. You may have heard of Einstein's formula E=mc2, which basically means that mass is the equivalent of energy. So you can convert mass into energy or energy into mass. But you can't destroy the total amount of mass and energy together. So the total amount of energy-mass stuff, that can't be destroyed. And that is always somewhere.

John - So everything that has ever been created in nature, is still in existence somewhere in some form?

Dave - Well, all of the energy in the universe is still there.

Chris - It originated in the big bang, which was the unleashing of a huge amount of pent up energy that existed in a tiny amount of space that was enourmously dense, and minute.

John - I'm thinking of matter created on earth like an acorn grown into an oak tree.

Chris - Well those are all atoms and those are all molecules, which are built from atoms, and the point is that all of the things and all of the complex elements you see here on earth must have been made in a star somewhere else in the universe. Nothing that was produced in the big bang came out more complicated than Hydrogen, Helium, and a little bit of Lithium. Which are the first three elements of the periodic table. And more complicated things were produced in stars like our own sun and bigger stars that very quickly ram things together and make very complicated elements which when the star blows up, are disseminated all around the universe. And then they gather together to make a new star, and new planets and things like our solar system.

I'd say probably it's not true. Especially if you have a tall building or something pointy on top of a mountain it actually is much more likely to hit something that it's struck before. Because it tends to hit sharp things. The empire state building that's in New York in America got hit 15 times in 15 minutes a few years ago. So there's evidence that lightning certainly does strike the same place far more than once. 15 times, in fact. Because it's always trying to find the easiest way down to earth. And if there's an easy way made handy, it will use it every time.

Kat - I never realised this but aeroplanes can get struck by lightning. I had to fly to Glasgow and a friend of mine was on the next flight and got struck by lightning! Crazy!

Chris - It happens with a frequency of about once in every 10 years, in an aeroplane's lifetime. A friend of mine flew to America. He was already late for his flight, he finally got on the aeroplane, it was delayed landing by an hour and a half because the storm was so bad around Chicago, and then his plane got hit by lightning, and then it got hit by lightning again, so twice in the space of about ten minutes! And then they did land, and because all the flights had been grounded there were no hotels. So he ended up having to spend the night in a five star hotel which was the only thing that had any rooms left over and it cost him about 500 quid. Then he finally did get his connecting flight to the conference he was going to, and he was so late he missed his talk.

The correct answer is that the plants that grow them are actually clones. So instead of growing them from seeds, they're grown from cuttings, so from existing plants. So obviously the first seedless grapes were a plant that arose through mutation, that means that they don't have seeds. And some growers must have noticed this. And you can basically take a little shoot or a stem off the plant, dip it in rooting powder, put it in the ground, and a new tree will grow. This is how a lot of plants are cultivated now, and also a lot of seedless varieties. It's causing problems now with bananas though. Because they're all clones, they're getting struck down by funguses and things. If a population is genetically identical, it can very easily be wiped out because it has the same resistance to different pathogens.

This is a bit dodgy. Scientists have looked at whether champagne makes you tipsy more quickly. There was a piece of research done at the university of Surrey by a researcher called Fran Ridout, in about 1999. They got all the researchers in their laboratory one week to drink some still champagne that had been flattened by leaving it open for a while. Then a couple of weeks later they got them to drink the same amount of fizzy champagne. And after each drinking session they got them to do some simple tests to work out how well their brain was working. And they found that they were working less effectively after the fizzy drink, than after the non fizzy drink. But then of course there's a big placebo effect here because you know it's fizzy and so you're more likely to think that you're getting more drunk.

Well those types of muscles are what are called smooth muscle. And this is different to the parts of the body which you move around voluntarily, things like your arms and legs. They have a slightly different form of muscle. The answer is that they can't necessarily spontaneously repair unless you give them a bit of help. What scientists are now finding is that there are certain stem cells that we can inject into people. And those stem cells come out of the blood, they go into the muscle, and they then turn into muscle cells, and they can repair. And there's a scientist who's working in Italy, who published a paper recently when they were looking at muscular dystrophy, which is a disease of muscle in the legs and the arms, skeletal muscle. And they managed to steal some stem cells from the walls of blood vessels, put them into the blood stream of some dogs that had muscular dystrophy, and the dogs got better. Because the cells they injected went into the muscles and repaired them. But in terms of repairing the bowels, it depends why there are problems with the muscles in the bowels. There are a number of disorders, which can affect your intestines, and it depends which one you've got, and whether or not it's possible to put it right. There's one disease which is called Hirshprung's disease and in this one, for some reason you don't get proper nerve connections formed along the bowels, and so messages which tell the bowels to push food along don't get through properly. So the bowels don't work very well. That's very difficult to repair unfortunately. But there are some disorders where, if there is a bit of the bowel which isn't working properly, by taking it away, and stitching the two ends together, you can overcome the problem. Your bowel has a hell of a lot of nerves in it, it's got as many nerves as parts of the brain in fact, and it can actually learn. That's why we have a bowel habit. Your bowel actually learns what your daily rhythm or pattern is all about.