Perms

Perms often involve the use of chemicals that break and reform your hair to create a new shape of your hair. Your hair is washed and wrapped on a perm rod. This solution creates a chemical reaction that softens the inner structure of the hair by breaking some of the cross links within and between the protein chains of the hair. The hair swells, stretches and softens, then molds around the shape of the perm rod.

http://www.beauty.about.com/od/hairbasics/a/perms.htm