Fig Khalaf, age 10, of Maplewood, N.J., for .s question:

 How much of the human body is water?

A person can survive for several weeks without food, but for only a few days without water. Every day, we normally drink a large quota of water, just plain or with other liquids such as milk and coffee which are mostly water. We even eat water with our food. For lettuce and cabbage are 90 per cent water, an egg is 70 per cent water and even a slice of bread is 35 per cent water. Every day we also lose a quota of water through the kidneys, as vapor breathed out by the lungs, as perspiration and maybe as tears.

The body needs a constant supply of water to keep going, for every day we drink a certain amount and lose a certain amount. The human body is made of billions of tiny cells, too small for our eyes to see. These tiny cells work together to perform a million miracles. Some help us see, some help us hear, others help us talk and sing. The heart beats, the lungs breathe, the brain thinks, all because the body's cells are performing their many duties.

Each living cell is filled with a magic fluid called protoplasm, most of which is water. The blood stream is mostly water and so are the cells of the brain. Even the cells of the sturdy skeleton are from 35 to 50 per cent water. In fact, water accounts for between 65 and 70 per cent of the entire weight of the body. If you weigh 100 pounds, about 66 pounds or roughly eight gallons of this body weight is plain water.

Water, of course, is a simple chemical, a compound of oxygen and hydrogen. Because of the wonderful miracles it performs, we might expect the body to be made from rare and strange substances, but this is not so. Most of it is made from common chemicals similar to those found in the earth's crust.

However, many substances are compounds made from molecules of assorted atoms and some of these molecules are so large and complex that science has not yet mastered their secrets.

A compound, however, is made from atoms of the basic chemical elements, just as water is a compound of the elements oxygen and hydrogen. Suppose we could break up these many compounds into separate atoms and weigh them. We would discover that 99 per cent of the human body is made from just six common elements such as oxygen and carbon. There are'also small amounts of potassium, sulphur and a few other common elements and traces of copper, zinc and a few rarer elements.

The blood stream is always circulating, protoplasm is forever changing. Old cells are forever wearing out and being replaced with new ones, The living processes of the body are always changing and moving, and much of this shifting and flowing depends upon the supply of water streaming here and there throughout the cells of the entire body.