And those are ridiculously easy once you get the trick. The first ones that were out, I remember, hit in the very early 90's in magazines like Popular Science. In those, you had two dots right above the picture, and the instructions were to visually split the dots and combine them in such a way there were now three. That achieved, look down at the picture.
Most, if not all of the later ones removed those instructions, and it became a game of "stare at this until you get it." For the really old-timers, we understood the real trick is to cross your eyes slightly and vary the degree until you hit the magic separation necessary to resolve the picture.
I'm not entirely convinced the effects are related to those used for 3D movies, since Magic Eye works through the transposition created by overlapping the two visual fields by crossing your eyes.