Mary Clare Gergen, age 10, of Milwaukee, Wisconsin, for her question:

How can the sun tan the skin?

Sunshine teems with all sorts of invisible energy. You cannot see this energy, but it does things to the tiny molecules in certain substances. The shin is made of molecules arranged in layers of little cells. On the top is a paper thin layer of dead cells called the epidermis. Under the epidermis is the true skin called the derma. The derma layers are made of living cells, with nerves and blood vessels. They also have hair roots, sweat glands and the stuff for making sun tans.

Below the surface of the skin there are small factories made of living cells. Some of them make a brownish material called melanin. Pale people have dust enough melanin to show through. People with more and still more melanin have brown or black skins. Usually it does not color the surface layer of the shin. But the energy of warm bright sunshine makes it multi¬ply. Then millions of molecules of melanin fill the deeper cells and spread up to the surface cells. The skin is tanned with brown melanin.