"There are more animals living in the uncleaned matter of the teeth in one's mouth than there are men in the whole kingdom," wrote Anton van Leeuwenthoek, the inventor of the microscope, in 1683.

Today this "uncleaned matter" is known as plaque, the tenacious gooey film of bacteria that forms constantly on teeth. Plaque combines with sugars to form acids that attack the tooth enamel and eventually cause cavities and gum disease.

The two most effective weapons against plaque -- besides the toothbrush and dental floss -- are fluoride, a mineral added to drinking water, and sealants, thin plastic coatings applied to vulnerable teeth.

"With the two together, you have total protection of the tooth surface," says Jacqueline Clarke-Martin, assistant professor of pedodontics at Howard University College of Dentistry.

No one knows exactly how it works, but experts agree that fluoridation of the public water supply is the biggest factor in the dramatic decline in tooth decay in children.

Fluoridation reduces the incidence of cavities by about 65 percent, says the Public Health Service. First tried in 1945 in Grand Rapids, Mich., fluoridation is safe and cheap -- averaging about 30 cents per person per year -- according to the American Dental Association.

Tiny amounts of fluoride occur naturally in most water supplies, but usually at levels too low to guard against cavities. Residents of the District, Northern Virginia and the Maryland suburbs are among the 123 million Americans who live in areas where the water has been fluoridated to optimum levels.

Although fluoride reduces the risk of tooth decay on all tooth surfaces, it is most effective on the smooth surfaces. On the rough chewing surfaces, whose microscopic grooves and cracks are ideal breeding grounds for bacteria, sealants may be necessary.

Early use of sealants showed mixed results. But a consensus panel at the National Institutes of Health a year ago concluded that the latest sealants are safe, "highly effective" and "underused."

Sealants are applied to the chewing surfaces of children's back teeth. The treatment is painless. Teeth are cleaned, etched with a mild acid and coated with a quick-hardening liquid that seals out decay.

Sealants are used in children because teeth are most susceptible to decay two to four years after they cut through the gum. Even baby teeth are sealed, because if they are lost before age 12, permanent teeth may grow in out of position, causing an uneven bite and, possibly, expensive orthodontic work.