Boing Boing have a link to a really cool optical illusion put together by a guy called John Sadowski. It is a picture of a Spanish castle.
Leave your mouse pointer off the picture area and the picture is displayed in an orange and blue negative. Stare at the black dot in the center for 30 seconds and then while still staring at the picture move your mouse pointer over the image. It now changes to a positive black and white image. Except that's not what you see. Your eyes will see the image in color until you move your eyes. I thought it was a trick but it really works.
The guy promises to post instructions on how to do it with your own photos as soon as his bandwidth usage settles down.
[via Boing Boing]