Every person’s hair is slightly different in texture, color and shape. Hair is made up of a protein composed of long chains of organic compounds called amino acids. Some of these amino acids contain sulfur atoms. Sulfur atoms on one amino acid chain can bond with sulfur atoms on another amino acid chain.

The more of these bonds there are in a strand of hair, the curlier it is. No one knows exactly how this sulfur bonding makes hair curly. The bonds may change the shape of individual hair molecules, folding them back for an overall curly effect. Scientists think that one or two genes determine the number of sulfur bonds that form in your hair, but there may be other genes that control hair curliness as well. Hormones, and even the shape of the hair follicle, may also play a role.

You may notice that your hair gets curlier on rainy days. That’s because the amino acids that make up hair also contain hydrogen atoms – which can bond with the hydrogen atoms in moist air. The result? Frizzy hair.