A tooth is a hard structure, set in the upper or lower jaw, that is used for chewing food. Teeth also give shape to the face and aid in the process of speaking clearly. The enamel that covers the crown (the part above the gum) in each tooth can be broken down by acids produced by the mouth for digestive purposes. This process is called decay. To prevent decay, good oral hygiene, consisting of daily brushing and flossing, is necessary. The hardest substance in the human body is one of the four kinds of tissue which make up the tooth. It is enamel and covers the crown (area above the gum line) of the tooth. A bony material called cementum covers the root, which fits into the jaw socket and is joined to it with membranes. Dentin is found under the enamel and the cementum, and this material forms the largest part of the tooth. At the heart of each tooth is living pulp, which contains nerves, connective tissues, blood vessels and lymphatics. When a person gets a toothache, the pulp is what hurts.