Why does a dentist have to drill a tooth cavity?

The dentist has to drill a tooth cavity because the cavity (the hole in the tooth) is not just an empty space -- it is actually filled with decayed tooth material. In order to correctly prepare the tooth for a filling, the dentist must remove this decayed material. Once the dentist uses the drill to clean up the cavity, he or she can fill in the clean hole with either an amalgam filling (containing mercury, copper, silver, and other metals) or a composite resin filling.

When someone gets "plaque" on their teeth, they are getting a "film of bacteria." These bacteria stick to teeth. The bacteria then create acid as a by-product of their metabolism. That acid can eat through the tooth's enamel causing a cavity -- a hole through the enamel.

A tooth has three parts: The outer, very hard enamel, the inner, much softer dentin (think of dentin like ivory) and the core pulp, which consists of nerves and blood vessels.

Once acid breaks through the enamel, bacteria can invade the dentin and will eat through it fairly quickly. If the bacteria make it to the pulp, it causes an infection. You will need a root canal.

When dentists drill into teeth, they are shaping an opening. This is to ensure all of the rot in the dentin is removed entirely. Another reason for drilling is to create a shape for the filling to bond and fill the cavity completely. Fillings can be either a silver/mercury amalgam or they can be plastic.