Magic Eye Patterns
Magic Eye Patterns were pretty much a page of random patterns that looked like static on the TV. However, if you looked at them the right way (or lied) you could see a 3D picture.
The patterns were found in magazines and books and on posters and there were a lot of people around telling you how to see them. I remember sitting with your nose to the page and slowly moving it away from you, putting it behind glass or simply staring at it for several hours until your eyes got tired. But I think the most commonly used way was to look on the page that told you what you should be seeing and lie.