Porcupine quills are as sharp as needles, detach very easily, and will remain embedded in an attacker. Unlike needles, however, the quills of porcupines have microscopic, backwards-facing barbs on the tip that catch on the skin making them difficult and painful to extract. Quills are about 75 millimeters (3.0 in) long and 2 millimeters (0.079 in) in width. If a quill becomes lodged in the tissues of an animal, the barbs act to pull the quill further into the tissues with the normal muscle movements of that animal, moving up to several millimeters in a day. Predators have been known to die as a result of quill penetration and infection. Quills are still capable of penetrating animals after death.