Sandra McGillicuddy, age 12, of Visalia, California, for her question:

What causes tooth cavities?

By the time you reach your teens, you should have 32 permanent teeth    and they are expected to last to the end of your life. But sad to say, this durable plan rarely works. Almost everybody gets a few tooth cavities and all sorts of trouble may attack the gums. Many of these problems can be prevented and naturally a sensible young person wants to know how.

Dentists cannot point to a single cause of tooth decay. But they can tell us a number of different things that seem to work to avoid it. Basically, a cavity occurs when decay bacteria destroy the tough enamel coating of a tooth. From there they attack the dentin core and the pulpy interior. The surface damage to the enamel is painless and you don't notice it. But when the cavity reaches down into the dentin, the tooth hurts when it touches something hot or cold. When the trouble digs down to the nerves and blood vessels in the pulp, the poor tooth begins to ache with a vengeance.

This is one reason why a sensible person visits the dentist every six months or so. The dentist is an expert at detecting a young cavity before it digs down to sensitive stage two and agoniain¢ stage three. The surface patch of decay can be cleaned out and sealed from further damage    almost painlessly.

The villains of this toothy story are bacteria, too small to be seen. The warm, moist inside of the mouth is just the sort of place where bacteria thrive and multiply. Many types of bacteria live there and most of them do no harm. A few are helpful and a few types are destructive. Most of them thrive on particles of food that get trapped between the teeth and around the gums.

Enemy bacteria break down food particles and deposit crusty layers of plaque around teeth and gums. This fermented acid material may inflame and infect the gums, right down to the jaw bone. It also may help the decay bacteria to start their destructive work on a tooth.

This is the second reason why a sensible person makes regular visits to the dentist. When the cavities, if any, are filled, the dentist removes the accumulation of plaque from around all the teeth. This removes swarms of destructive bacteria and reduces the chances of future cavities. Between visits, naturally we tend to our home tooth care. We use tooth paste and a softish brush to give them a daily scrubbing and dental floss to roust food particles from between the teeth.

However, it is a fact that some people get more cavities than others, no matter what they do. This is partly because some of us inherit better teeth from our ancestors. But teeth, of course, are built from the foods we eat and it stands to reason that diet makes a difference.

Teeth are made mostly from calcium and phosphorus, which are plentiful in milk and leafy green vegetables. However, we need a well¬rounded, balanced diet because the whole body must be healthy to build sturdy teeth. And some foods work against the teeth. People who nibble sugary snacks and sip endless sweet soda pops are more likely to get cavities than those who don't.