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How does an Etch a Sketch work?
The inside of the glass screen is coated with aluminum powder. When it is scraped off by the movable stylus, a dark line remains on the light gray screen. The line is actually just the darkness on the inside of the case. When the toy is turned upside down and shaken, the styrene beads inside smooth out and re-coat the inside surface of the screen with aluminum powder, causing the image to disappear.
There are several practicing artists who make professional, lineographic works with Etch a Sketch as their medium. By removing the aluminum powder then resealing the device, they make their masterpieces permanent.