Mammals have a unique arrangement of a lower jaw directly hinged to the skull. The upper jaw is fixed, while the lower jaw is movable. A powerful bite results which, coupled with specialized teeth, allows mammals to eat a diversity of foods. Straight-edged incisor teeth at the front of the jaw are for cutting and gnawing. Pointed, canine teeth on the sides can grab and tear meat. Flat, broad molars are used to grind or crush plants or shelled animals. Mammals tend to have more than one type of tooth (heterodont dentition), with the mix depending on diet. Meat-eaters have sharp incisors and canines plus slicing molars; mammals who eat tough plants have duller incisors and canines, but large, ridged molars. All types of mammal teeth are replaced once or never during a mammal's lifetime. A coating of enamel, the hardest material found in a mammal's body, makes teeth built to last.