Michael Weinstein, age 9, of Sarasota, Fla., for his question:

HOW DO TEETH DECAY?

Daily brushing of the teeth is perhaps    the easiest and most effective step you can take to prevent tooth decay. In addition to keeping your teeth clean, daily brushing is also an excellent way to stimulate gums and keep them firm. Brushing should be done after every meal if possible, or rinsing with water if a brush isn't handy when you eat away from home.

Dentists don't know exactly why teeth decay, but they certainly know how teeth decay. Bacteria found on the teeth causes food to ferment, and fermentation forms an acid that destroys tooth enamel. Once a tooth's enamel has been destroyed, decay can attack the dentine, and unless the cavity is filled quickly, the decay can progress.

Dental caries, as tooth decay is called, is the most common disease of man. Almost everyone has a decayed tooth at some time during his life. The shocking statistic is that about half of all the 3 year olds in the United States have at least one decayed tooth, while more than 96 percent of all high school students have some decayed teeth.

Cavities, which are holes in the teeth caused by caries, usually form on the chewing surfaces of the back teeth and near the places where the teeth touch each other. Decay happens at these places because it is often ha=d to clean them properly.

In addition to daily brushing, a person should make a habit of visiting his dentist twice each year. with this type of schedule, a dentist will often be able to remove small decayed areas and fill the cavities painlessly. This will prevent later tooth loss.

Also, a well balanced diet, most dentists agree, can help a person develop and maintain sound teeth. Since teeth are made up largely of phosphorus and calcium, foods containing these items are important. You'll find rich amounts of these elements in milk and leafy green vegetables.

Fluoridation has been a controversial subject in the area of tooth care. Many dentists believe the chemical should be added to drinking water as a way to prevent tooth decay, while others are against its use. Fluorides are included in some toothpastes and many dentists use the treatment of painting a fluoride solution on children's teeth.'

Many dentists recommend salt and baking soda as an inexpensive and effective dentrifrice. Some suggest mixing one part of salt with three parts of baking soda, although either may be used alone.

An inflammation of the gums called gingivitis can be caused by food becoming packed between the teeth, improper tooth brushing, injury to the teeth or diseases that affect the body as a whole. The direct cause of the inflammation is the formation of plaque and tartar on the teeth. If gingivitis is not checked, a more serious condition called pyorrhea may develop. Loss of teeth can result