Q) Do fish sweat? If not, how do they keep cool?

A) Fish do not sweat, unlike mammals, which live surrounded by air and cool themselves by evaporating moisture that they've sweated from their skin. Fish have no such means of maintaining a constant body temperature and their blood is usually at the same temperature as their surroundings. Although the temperature range tolerance of some fish (such as rock pool dwellers) is remarkable, most fish will die if the water becomes too hot, too cold, or changes temperature too suddenly. This is why care is needed when transferring fish into a new aquarium - so that they have sufficient time to adapt to the temperature of their new surroundings.

Q) Why don't electric eels electrocute themselves?

A) The reasons for electric eels' immunity to their shocks are not fully understood. Electric eels produce a current that runs from tail to head in the fish and in the opposite direction in the surrounding water. It is thought that they are protected from a short circuit by the insulating properties of their skin and the tissue surrounding the nerves. But it has been observed that if their skin is broken, the fish do show signs of distress from the effects of their own electric discharge.

Q) Why do you get a headache between your eyes at the top of your nose when you drink something cold?

A) All headaches are essentially caused by changes in blood flow in the head. As blood vessels open up and close they can trigger pain receptors. Headaches induced by eating something cold are officially called ice-cream headaches. When you eat something, cold blood rushes to the cold area to heat it up. To warm your mouth blood is diverted from the forehead and as the blood vessels there swell up, they trigger the pain receptors - giving a sharp pain at the top of the nose.

Q) Why are there only two sexes?

A) The presence of two sexes in a population is the most evolutionary stable of all systems. Three possible sexes would soon be reduced to two if a mutation occurred in one of them, allowing it to reproduce with only one of the others. The third sex would then die out. Quite why there are two sexes and not one is still a mystery, but in a changing environment two sexes are better at shuffling the gene pool to keep one step ahead of the latest disease.

Q) How much water does a tree drink?

A) Trees take in water at their roots and then lose it by evaporation through their leaves. A mature deciduous tree is thought to "drink" 50,000 litres of water in a year. On a warm windy day a tree can take up 2,000 litres of water - that's about 6,000 drink cans of fluid a day. It is estimated that 60 per cent of the water falling on a beech wood is returned to the atmosphere through the trees.

Q) Bird body temperatures are usually around 40 degrees centigrade - but they produce sperm at 35 degrees centigrade, so how do they keep their testes cooler than the body?

A) They don't! Birds only produce sperm at night when their body temperature drops by about 4-5 degrees centigrade. To stop it overheating the next day, they shunt the sperm into a kind of external lump called the cloacal projection - which outside the body is kept a little cooler at about 4- 5 degrees below body temperature.

Q) Why do we stretch?

A) There are several hypotheses about why we stretch. Firstly, when we are tired stretching uses muscles that draw the rib cage up and out - allowing the lungs to expand more fully - drawing in more oxygen. Stretching is also often accompanied by yawning, which is another strange phenomenon. No one knows the true reason for yawning - but it could also be to take more oxygen deep into the lungs. When we have just woken up, stretching may also be a way of warming the muscles ready for action by increasing the blood supply to them.

Q) If you can't get sunburnt through glass - by sitting in a car, for example - how can you get a tan from a sun-bed when the UV lights have glass around them?

A) Sun-beds have specially designed glass which doesn't have any iron impurities in it. Car windows deliberately have about 0.1 per cent iron metal in the glass to stop the plastic inside the cars from deteriorating. These metals reflect the UV light frequencies rather than transmitting them through the glass.

Questions and answers are provided by Science Line. You can use its Dial-a-scientist service on 0345 600444.

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