Randy Gale, age 15, of Lemon Grove, California, for his question:

What percentage of the human body is water?

If every gram of moisture were evaporated from the body of the 200 pound man, the dehydrated husk would weigh about 80 pounds. Naturally this ghastly misfortune would be fatal. Every living cell needs water to carry on its personal activities. Water is also essential as a transportation system in the vital teamwork between tissues and vital organs. The percentage of water for various biological functions must be so much and no more. Several complex systems operate continuously to keep these percentages in delicate balance.

The average human body strives to maintain a water content of about 60 per cent. A chubby body, padded with layers of surplus fat, contains a lower percentage than normal. This is because the percentage of water in most fatty tissue is about 20 per cent, considerably lower than that in most tissues. The wateriest tissues are in the brain    which may come as a surprise. The water content of the brain's white matter is 70 per cent. The surface gray matter, where the thinking is done, is 85 per cent water and    so are the nerves. These tissues must be doused in moisture in order to transmit electrochemical signals throughout the body.

Since the blood is liquid, one might expect it to be the wateriest of the body's tissues. True, its content is 80 per cent, which is 20 per cent above the total average. But the nerve tissue and the brain's gray matter are wetter. One might expect the long lean cells of muscular tissue to be drier than average tissues. Actually their moisture content is about 75 per cent. They too require water to conduct fast traveling signals.

The skin feels fairly dry to the touch. But this is merely the thin surface layer above various layers of softer cells. The water content of the total skin is 70 per cent, the same as that of the liver. The kidneys require a water content of 80 per cent to carry on their sifting and drainage operations. Hair and nails are made from dead, inactive cells and their moisture content is not worth mentioning.

One might suspect that the bones contain little or no moisture because they are built from hard, durable cells. But the core of a long bone is stuffed with nerves and blood vessels embedded in active living cells. The average water content of the skeleton is 25 to 30 per cent. This list of contents is incomplete without the connective tissue that both links and partitions all the cells throughout the body. This all important network has a water content of 60 per cent, the same as that total content of the average body.

A living human body, of course, is above price. So the fact that 60 per cent of it is ordinary water may come as a let down. What's more, all of its priceless ingredients are made from common chemicals. Oxygen and carbon, hydrogen and nitrogen, calcium and phosphorous make up 99 per cent of its multitude of miraculous biological substances. The remaining one per cent contains potassium and six other ordinary elements, plus traces of more than 20 more. These common chemicals are transformed into vital biological substances that require delicately balanced solutions to carry on their functions.