Fluoride exists naturally in water sources and is derived from fluorine, the thirteenth most common element in the Earth’s crust. It is well known that fluoride helps prevent and even reverse the early stages of tooth decay. Tooth decay occurs when bacteria – found in the plaque that dentists try so hard to get rid of – break down sugars in food. This process produces damaging acids that dissolve the hard enamel surfaces of teeth. If the damage is not stopped or treated, the bacteria can penetrate through the enamel to the underlying tissues of the teeth, causing cavities (also called caries). Cavities weaken teeth and cause pain, tooth loss, or even widespread infection in the most severe cases. Fluoride combats tooth decay in two ways. It strengthens tooth enamel; a hard and shiny substance that protects the teeth, so that it can better resist the acid formed by plaque. Fluoride also allows teeth damaged by acid to repair or remineralize themselves. Fluoride cannot repair cavities, but it can reverse low levels of tooth decay and thus prevent new cavities from forming.