Sandra Kerka, age 10, of Cleveland, Ohio, for her question:

What are fingernails made of?

All parts of the body are made from small units called cells. There are special cells for building nerves and other kinds for building muscles, f1esh, bone and skin. The cells used to build fingernails and toenails are very much like those used to build the strands of your hair. Unlike the cells of the flesh, the nail cells have hard and horny walls.

Nails are different from flesh in other ways, also. Flesh is living material, fed by blood vessels and riddled with networks of sensitive nerves. Fingernail material is made from dead dry cell walls with no blood vessels or sensitive nerves. This is why we feel no pain when the nails are cut and trimmed.

Laura Whybrow, age 10, of San Francisco, Calif., for her question:

What are our teeth made of?

Your pearly teeth are made from calcium and other chemicals found in milk and wholesome cheese, in nourishing meat and eggs, from vitamin chemicals found in green and yellow vegetables and from different minerals found in water and a wide assortment of foods.

A person grows two sets of teeth during a lifetime. The first tooth cutting begins when a baby is about six months old, and at the age of two and a half the young person has a full set of 20 milk teeth. Then the body begins to absorb or dissolve and take back some of the materials used to make these first rather soft teeth. Most of the roots disappear and the teeth fall out one by one. The second set of teeth begins to Erupt through the gums when a young person is about six years old.

The second set is the permanent teeth that should be durable enough to last a person all through his or her lifetime. The quality of these teeth depends upon the ingredients from which they are built. And these ingredients, of course, are extracted from the foods we eat. Your wonderful body knows just how to extract molecules of lime and calcium from the milk you drink and exactly how to use the vitamins and other chemicals in every helping of salad and leafy vegetables. It also takes fluorine and other minerals from water for its tooth building work.

Your permanent teeth are built during your grade school years. This is the time when sensible young persons dine daily on a balanced diet of tooth building, bone building foods. For those teeth and bones must last a lifetime. Sugary treats are bad for teeth and cause cavities, and crusty tartar may cause decay. So cut down on sweet treats, brush regularly to remove decaying scraps of food and visit the dentist who will remove the tartar and check for cavities.

During the tooth building years your body performs daily miracles to extract the right ingredients from your diet. From limy calciums and traces of assorted minerals and metals it manufactures molecules of durable enamel to coat each tooth. The enamel covers the crown of dentine that is made from softer, pulpy material. In the center of each tooth is a living core of flesh with blood vessels and bearing sensitive nerves. All the ingredients for making these different parts of the teeth are taken fragment by fragment from your food.

Sometimes sickness prevents the body from building healthy teeth, and then a person may have weak teeth through no fault of his or her own. But as a rule we get the kind of teeth we deserve. To a large degree the quality of permanent teeth depends upon the diet during school years. Soft starchy foods yield little or no tooth building materials. Sugary sweets and soft drinks encourage decay. Teeth and gums also need chewing exercise and plenty of brushing to remove trapped scraps of food.