In the adult mouth, there are up to 32 permanent teeth, 16 in each jaw. Young children have only 20 primary teeth, which are replaced by the permanent teeth. The main function of your teeth is to break up food into fragments small enough for you to swallow and digest. Human beings are omnivores—that is, they eat both animal and vegetable food. Human teeth thus have two basic shapes.
The six front teeth in each jaw have single, sharp edges, like knives. The edges of the four incisors are straight, for cutting off pieces of food. The edges of the two canines come to a point, for tearing food as well. Each of these teeth has a single root.
The ten back teeth in each jaw—five on each side—have wide chewing surfaces, to grind food. Each chewing surface contains two or more low mounds, called cusps, separated by hollows and grooves. On each side of the jaw are two premolars (sometimes called bicuspids) and up to three molars. The premolars are smaller than the molars, and have one or two roots. The upper molars usually have three roots; the lower molars have two or three. The additional roots brace these teeth against the heaviest pressures of chewing.
The parts of your teeth most susceptible to decay are the chewing surfaces of the back teeth, surfaces where adjacent teeth meet, and surfaces nearest the gumline.