Beavers have massive skulls adapted for withstanding the forces generated by their powerful chewing muscles. Their four chisel-shaped incisors grow continuously. The incisors' outer enamel is very thick and colored orange due to the presence of iron compounds. The roots of the lower incisors extend throughout the length of the lower jaw. Beavers have one premolar and three molars on each side of the upper and lower jaws, 20 teeth in total. The molars have meandering ridges for grinding woody food. The eyes, ears and nostrils are arranged so that they can remain above water when the rest of the body submerges. The nostrils and ears have valves that close underwater while nictitating membranes cover the eyes. Unusual among mammals, the epiglottis is contained in the nasal cavity rather than the throat, preventing water from flowing into the larynx and trachea. In addition, the back of the tongue can rise and create a waterproof seal. A beaver's lips can close behind the incisors, allowing for chewing in water.