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Q Sunlight seems to be more powerful and more capable of burning human skin in the past 20 years. Have specific dangerous wavelengths in the spectrum increased in intensity recently?

A If the energy output of the sun increased, there would be no reason for all wavelengths not to increase to the same extent. It is therefore unlikely that some wavelengths have increased in intensity more than others. The higher incidence of skin cancer is more likely to be caused by our changing social habits: we sunbathe more than we did and go on holiday to sunny countries. Although the protective ozone layer in the earth's atmosphere is getting thinner as a result of pollution from CFC gases, the "holes" have been found only over Antarctica (and to a lesser extent the Arctic) and there is no evidence as yet linking the thinning over temperate latitudes with the increased incidence of skin cancer.

Q Why do some cars have a static build-up and others don't?

A It is thought to have something to do with the carbon content of the car tyres. Tyres with high carbon content are more likely to dissipate static electricity.

Q Why is nitrogen used in so many explosive materials if it is so inert?

A Nitrogen gas is inert because there are three bonds holding together only two molecules. If you like, it doesn't have any hands free. In explosives, the nitrogen atoms are held to other atoms quite loosely and can therefore form other bonds readily. This makes them an unstable material.

Q Who discovered black holes?

A Pierre Laplace, the 18th-century French philosopher, first suggested the existence of a gravity field so strong that light could not escape. Albert Einstein did the mathematics and John Wheeler coined the term "black hole". As yet there is no absolute proof that a black hole exists, although there is very strong evidence suggesting it does.

Q Why isn't there a cure for the common cold?

A The common cold is caused by many different viruses. At the moment we have few medicines to treat virus infections. Those which do exist are far too strong (and have potentially unpleasant side-effects) to risk using them just to get rid of a cold. There is no vaccine for the common cold because so many different viruses cause it and it is impossible to cope with them all. A cold makes you feel a bit grotty for a few days and then you can fight it off without treatment. So a "cure" isn't really needed.

Q How do bees navigate?

A Bees navigate using the position of the sun in relation to their journey. This information is conveyed to other members of the hive through a dance that the bees do.

These questions and answers are provided by Science Line. You can use its Dial-A-Scientist service on 0345 600444