Fluoridation is the addition of a chemical called fluoride to water supplies to help teeth resist decay. In the 1930's, researchers discovered that people who grew up where water naturally contained fluoride had up to two-thirds fewer cavities than people living in areas without fluoride in the water. Newburgh, New York, and Grand Rapids, Michigan, began to fluoridate their water in 1945, as an experiment. By the 1950's, the tests showed that the incidence of tooth decay had decreased in these cities, and United States public health officials recommended fluoridation for all communities.