Etch-a-sketch

Do you know how an Etch-a-Sketch works? I didn't know until Travis and Trouble broke one. (Which Travis, of course, turned it into a science lesson!) There are these little balls covered in some kind of silver element. When you turn the Etch-a-Sketch upside down and shake it, the silver on the balls rubs off on the screen and gives the screen a silver look. Inside the toy is also a little metal piece that looks like a pencil tip. It is hooked to a bunch of wires, which are hooked to the white knobs. When you turn the white knobs it moves the wire that are hooked to the metal pencil tip, which scraps off the silver. Ingenious!!PS The silver washed right off!