Porcupine quills are as sharp as needles, can be removed very easily, and will stay stuck in an attacker. Unlike needles, however, the quills of New World porcupines have small, backwards-facing barbs on the tip that catch on the skin making them hard and painful to pull out. Quills are about 75 mm long and 2 mm wide. If a quill becomes put in the tissues of an attacker, the barbs act to pull the quill farther into the tissues with the normal muscle movements of the attacker, moving up to many millimeters in a day. Animals who try to eat porcupines have been known to die because of quill penetration and infection. Quills can still penetrate animals and humans even after death. The ancient belief that porcupines can throw their quills at an enemy has long been proved to be incorrect.