According to FutureDerm, curling irons work because their heat weakens the hydrogen bonds that holds the keratin making up the shaft of the hair. This weakening bond allows the hair's shape to change temporarily, becoming curly when a curling iron is used and straight when a flat iron is used.
Because the weakening of the hydrogen bonds in the hair's keratin is temporary, the curls produced by a curling iron are only temporary. Another factor that alters keratin is water; rain, showering or humidity can undo the work done by a curling iron. Heat from a curling iron also causes cracks in the hair's cuticle, which lets chemicals and sunlight penetrate the cuticle and damage the inner cortex of the hair.