Rabbits have long ears and long hindlegs for jumping. They have two pairs of upper incisors, the first enlarged and chisel-like, the second small and directly behind the first, lacking cutting edges. As in some other animals, the incisors continue to grow throughout life, but are constantly worn down by use. Rabbits are herbivores that make the most out of their food by eating it twice (reingestion). Vulnerable to predators, they evolved a protective mechanism of quickly filling the stomach, then hurrying to a hiding place, where they defecate pellets of soft, green undigested material, which they then eat and digest normally to obtain maximum nutrition.