Autostereogram

To see a random dot autostereogram correctly, draw the eyes (and face) towards, say, within one inch of the image, and then slowly pull away, until the central 3D shape coalesces into view.

If you stared at the pattern long enough, your brain would trick itself into seeing a hidden 3D image. It was called an autostereogram, and it was easy to make one if you were a moderately skilled programmer.