Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Tooth Decay ?
    ### Response:
    You call it a cavity. Your dentist calls it tooth decay or dental caries. They're all names for a hole in your tooth. The cause of tooth decay is plaque, a sticky substance in your mouth made up mostly of germs. Tooth decay starts in the outer layer, called the enamel.
    