Book 31, Question 43
Why do beaks keep growing?
A beak is made of bone covered with a tough sheath of keratin, the same material as your fingernails.
Like fingernails, the sheath keeps growing from the base. As the bird crunches seeds and wipes its beak on branches, the tip wears down. The two balance out, so the beak stays the right length and sharp. If the tip gets chipped, it can slowly regrow.