When hair is curled, it undergoes a temporary physical change in which hydrogen bonds are temporarily weakened and altered. This physical change is easily reversed when the curls are introduced to further heat or humidity.
When hair is heat curled, its hydrogen bonds are temporarily broken. Hydrogen bonds hold together hair's keratin, or protein. When this bond is broken, the hair can be rearranged into the desired curl pattern. However, hydrogen bonds are very weak and easily revert back to their natural state when they encounter heat or water. Some people prefer to roller set their hair to eliminate the risk of heat damage associated with hot curlers. Rollers achieve the same effect as heat curling by the same mechanism, breaking the hydrogen bond. As a result, roller-set curls revert back to their natural state just as easily as heat curls do.
For those who seek to permanently curl their hair, a stronger process is required to ensure that the hair remains curly. A perm, short for permanent wave, permanently changes hair from straight to curly. Perms work by chemically breaking and rearranging the hair's stronger disulphide bonds, thereby permanently transforming the structure of the hair from straight to curly.