When I was about 12. I don't remember where I found it, but the idea was that if you look very hard you would see a stereoscopic image. There is actually an image, but as soon as you get it in focus... "ARGH!" A dead guy pops and screams for about 2 minutes.
Result: one very shaky cross eyed 12 year old.
Result: one very shaky cross eyed 12 year old.