Have you seen this optical illusion? The image can interpreted either as a red Indian or an Eskimo entering a cave. I got to thinking on how you could make up optical illusions like that. Perhaps you could access a page of random Google images and turn down the monitor's brightness and contrast to a very low level. You would try to recognise the contents of each image you see - and maybe even draw a quick sketch. Then you would turn up the brightness and contrast levels to normal and see how you were wrong. Perhaps - if you can draw well - between your incorrect image and the correct image you could draw an image that could be interpreted as both.
Scary, Liar, Ships, Birds, Skull
And here's another one I like - it forms a different picture if you view it upside-down.