Why is some hair curly and some hair straight?

Hair mostly consists of a protein named keratin, which grows from the hair’s follicle. Keratin can also be found in the nail. In fact, up to 90 percent of the hair’s dry weight is keratin. Keratin has a great amount of disulfide which is responsible for the elasticity of hair. Based on the number of disulfide bonds between hair proteins in the hair shaft, the hair becomes curly or straight: The more the number of bonds, the curlier the hair would be!

The amount of humidity in the air not only makes for what some label a “bad hair day,” but alters the degree of curliness or of straightness of hair. This occurs when high humidity forces water back into the hair fiber, acts on its protein structure, and forces the hair shaft to return to its original structure.

You can temporarily change your hair from curly to straight or vice versa, but none of these changes would be permanent; Because the new hair gradually grows in as the treatment you used grows out.