A cavity, or tooth decay, is a small hole in the tooth that develops because of acid. This acid comes from bacteria that lives in the plaque on your teeth. This bacteria loves sugar, and when you eat sugar, so too does the bacteria. And it releases acid as a by-product (bacteria poop). That is why it is important to brush away the plaque and avoid too much sugar in your diet. And it is really important to get cavities fixed to prevent them from progressing.