Fluoride is a chemical compound that strengthens tooth enamel and helps prevent cavities. Your tooth enamel is the hard outer coating that protects your teeth, and fluoride works by making this coating more resistant to the acids that cause decay.

Most toothpaste contains fluoride, which is why dentists recommend brushing twice a day. Many communities also add small amounts of fluoride to their drinking water, a practice that began in the 1940s and dramatically reduced tooth decay across entire populations. When you visit the dentist, they might apply a fluoride treatment, a special gel or foam that gives your teeth an extra protective boost.

While fluoride is beneficial for teeth in the right amounts, too much can cause problems. That's why toothpaste tubes warn against swallowing the paste and why communities carefully measure the amount added to water.

Before fluoride became common, tooth decay was far more widespread and serious. Today, thanks largely to fluoride in toothpaste and water, children grow up with much healthier teeth than previous generations did.