Tooth decay is the destruction of your tooth enamel, the hard, outer layer of your teeth. It can be a problem for children, teens and adults. Plaque, a sticky film of bacteria, constantly forms on your teeth. When you eat or drink foods containing sugars, the bacteria in plaque produce acids that attack tooth enamel. The stickiness of the plaque keeps these acids in contact with your teeth and over time the enamel can break down. This is when cavities can form.