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Q Why does milk expand when it is cold?

A Milk is made mainly of water. Water has the curious property of being more dense as a liquid than as a solid. This means solid ice floats on liquid water. It also means that the ice takes up more space than the liquid and so milk expands when it freezes, pushing the top off the bottle.

Q Do identical twins have identical fingerprints?

A If the twins are monozygots (from the same egg), they will have identical fingerprints. Environmental factors and changes after birth, however, are likely to change them later.

Q Does the moon have a magnetic field? How would a compass react there?

A The moon has no overall magnetic field, but it does have local, randomly directed magnetic fields. What a compass would do would depend on where you were. In some places, it would register a field, in others it wouldn't.

Q Why does the moon keep one face turned towards us?

A It rotates on its axis once in 27 days and revolves round the earth in the same period of time. This is known as synchronous rotation, ie keeping the same face (the "near side") towards Earth.

Q Which planet has the hottest surface temperature?

A The temperature on Venus is 462OC due to its carbon dioxide atmosphere, the product of a runaway greenhouse effect.

Q How do flies land on a ceiling and take off?

A Flies turn over in flight as they approach a ceiling and hit the surface with either their front or back legs, and rotate on to the others. If they need to grip a rough surface, they use claws. If it is a smooth surface, they have sticky pads like doormats of sticky hairs with fluid on the end. To take off, they slide or peel off their feet from the surface.

Q When a limb is amputated, it often feels as if it is still there. Why?

A This bizarre phenomenon is called "phantom limb" pain. Sometimes, pain seems to come from the amputated part, which is probably due to stimulation of the stump. This happens because there are still nerves in the stump that would have continued into the leg or arm but have now been severed. Nobody is absolutely sure how it happens, but messages are still being sent along the nerves.

Q Why does hair go grey?

A A hair is a tube filled with cells and pigment with spaces between. When the hair is young, the spaces are filled with fluid and this keeps the pigment in place - your hair stays brown or blond or whatever. As we get older, our skin doesn't produce hair quite so well and they become filled with air. The pigment is lost and the hair gradually becomes white.

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