Why can’t you keep your hair straight, while you can with curls?

Answer

Hair mainly consists of fibers that are made up of the protein keratin. The fibers are elastic, and will spontaneously have a tendency to curl up (=curl). The keratin proteins hang together via a chemical bond between two sulfur atoms. When applying a perm, or stretching it to straight hair, the hairdresser uses a product that can break that bond and force the hair into an unnatural shape. However, the hair will return to its original shape over time, as this gives a more stable structure. When your straight hair pulls into curls, that will also return over time.