Quills are the stiff, sharp spines of certain birds, most notably porcupines and hedgehogs. These structures serve as a form of defense, allowing the animals to deter predators. When threatened, a porcupine can raise its quills, making it more difficult for attackers to approach. Quills are made of keratin, the same material found in human hair and nails. In addition to their defensive role, quills have been historically used by humans as writing instruments. The hollow shaft of a quill can be dipped in ink, allowing for smooth writing on paper. This practice was common before the invention of modern pens and is still appreciated in calligraphy today.