I had a (really creative) buddy growing up who figured out that you could do the wallpaper function in Mario Paint (which filled the canvas with repeating icons

I had a (really creative) buddy growing up who figured out that you could do the wallpaper function in Mario Paint (which filled the canvas with repeating icons) and then stamp over some of the icons but slightly offset, and when you crossed your eyes funny they’d pop out of the screen. Very easy DIY Magic Eye.