When it is close to hatching, a chick uses its egg tooth to pierce the air sac between the membrane and the eggshell. This sac provides a few hours' worth of air, during which time the chick hatches. When a chick is ready to hatch from its egg, it begins the process of "pipping"; forcing the egg tooth through the shell repeatedly as the embryo rotates, eventually cutting away a section at the blunt end of the egg, leaving a hole through which the bird may emerge.[2] Some species, including woodpeckers, have two egg teeth; one on both the upper and lower bill.[3] After time the egg tooth falls off or is absorbed into the growing chick's bill.[4]