Hedgehogs are easily recognized by their spines, which are hollow hairs made stiff with keratin.[5] Their spines are not poisonous or barbed and unlike the quills of a porcupine, cannot be easily plucked from the body. However, the immature animal's spines normally fall out as they are replaced with adult spines. This is called "quilling". Spines can also shed when the animal is diseased or under extreme stress