The human jaw holds four types of teeth including the incisors. Embedded in the so-called "central" section of the jaw, the four incisors, both central and lateral, are curved in order to facilitate biting into food. When the mouth is closed, the upper and lower incisors form a sharp closing that works like a pair of scissors to cut into food. The resulting cut-up pieces of food are then chewed by the other teeth. It is interesting to note that incisor roots have only one extension.