Here is a rather long, but very informative video showing a machinist opening up and analyzing the inside of the classic Etch-a-Sketch toy. He rambles a bit which I sort of enjoy. In any case, he's very knowledgeable about machines. The toy is also a small engineering marvel.
If this video has made you miss your own childhood toy, here is where you can get a new Etch-a-Sketch. Or...perhaps you want to take it apart now?