Sugar acts like an acid dissolving the enamel on teeth. Each time you eat a snack containing sugar, the resulting acid attack can last up to 20 minutes. The naturally-occurring bacteria in the mouth use sugar as energy to multiply and stick themselves to the surface of a tooth. Over time, this turns into plaque and continues to eat away at the tooth's enamel. Tiny holes will eventually be made in the enamel. These are cavities. Left un-treated cavities will continue to grow.