Your hair and fingernails are made of the same protein as bird feathers and reptile scales.

Keratin is a tough protein that builds the outer layers of your body. Humans produce a type called alpha-keratin. This protein contains a lot of sulfur. The sulfur atoms form strong chemical bonds called disulfide bridges. These bonds act like glue to hold the protein chains together.The hardness of the keratin depends on how many of these bonds are present. Fingernails have more bonds than hair, which makes them much stiffer. Hair grows from a follicle where special cells create keratin and then die. These dead cells leave behind the solid fibers that make up your hair strands.Fingernails grow from a spot under your skin called the nail matrix. The cells there flatten out into hard plates. Fingernails grow about 3.5 millimeters every month. Hair grows faster at about 1.25 centimeters per month.Other animals use different types of keratin for survival. Reptiles have beta-keratin to make their scales and shells hard. Birds use it to make feathers light and strong for flying. Keratin does not dissolve in water, which helps keep animals and humans from drying out.