Hair Dryer is an electro-mechanical device designed to blow cool or hot air over damp hair, in order to accelerate the evaporation of water particles and dry the hair. Blow dryers allow to better control the shape and style of hair, by accelerating and controlling the formation of temporary hydrogen bonds inside each strand. These hydrogen bonds are very powerful (allowing for stronger hair shaping than even the sulfur bonds formed by permanent waving products), but are temporary and extremely vulnerable to humidity. They disappear with a single washing of the hair.