What Are Cavities? Cavities are small holes that form in your teeth. They happen when bacteria in your mouth use sugar from foods and drinks to make acid. This acid damages tooth enamel, which is the hard outer layer of your teeth. Even though enamel is very strong, once it is damaged, it cannot grow back. How Cavities Form After eating, food and bacteria can stick to your teeth and form a sticky layer called plaque. If plaque is not cleaned away by brushing and flossing, t