Rodents are famous for their teeth. The large, paired cutting teeth (incisors) in the front of their mouth give them a buck-toothed appearance. The incisors are self-sharpening, thanks to the way they grind together during gnawing. While a strong enamel coating prevents wear on the front, the back of each incisor is soft and wears down. The outcome is a chisel-like shape that can cut into wood, food, or predators. The incisors grow continuously, creating tooth material for sharpening throughout the life of a rodent. A rodent skull is shaped to support the muscles used in gnawing, such as an enlarged region (the pterygoid) that anchors the lower jaw muscles. A skull modification allows the lower jaw to move not only up and down, but back and forth for better grinding action. Rodents are the largest group of mammals on Earth, and their ability to gnaw is part of the reason they have been so successful.