It was invented by Frenchman Arthur Granjean in the 1950's, who referred to it as l'Ecran Magique (i.e., the Magic Screen). The imaginative toy was licensed to the Ohio Art Company and in July 1960 debuted as Etch-A-Sketch. It was an instant success, as even the smallest child could begin making images immediately, and older kids could occupy themselves for quite some time producing artwork of increasing intricacy. (And everyone learned, or was reminded, that their painstaking work could be defaced or even erased completely by shaking or even giving a sharp jolt to the toy.) The toy was heavily advertised on television (a device which it not coincidentally resembled) and has sold tens of millions in the intervening years.
An Etch-A-Sketch works by virtue of the glass screen being coated on its underside by a mixture of plastic shards and powdered aluminum; the interior of the toy is a sealed chamber to hold the stuff. The knobs, as mentioned previously, move the metal stylus back and forth scraping the powder from the glass, producing the desired lines. Shaking or jarring the unit redistributes the powder to stick to the entire underside of the glass again, thereby erasing any masterpieces.
Children learn early on to make diagonal and even curving lines using the knobs, but this takes skill, patience, and a steady hand. Such masters of the 'Sketch have learned to photograph their finished works before a jostle or sneeze can undo their many hours of effort.