Fingernails, hair, and skin are made of a protein called keratin. But underneath, all these body parts have layers of living, growing cells. The live cells continuously divide, and by doing so, they push up the top layer of cells. These cells, which flatten and die, contain a lot of keratin. There are different types of keratin for different bodily structures, and the thickness of the layer contributes to the hardness of it. Bulls’ horns and birds’ feathers are made of keratin too!
Answer provided by Dr. Universe