I'm going to put these in here as they are health related. An optical illusion either tricks the eye (usually by fatiguing the color sensitive cones), or trick the brain. Feel free to post links to your favorites. As an aside, please DON'T put up screamers in here (an image that appears to be static but, after a set amount of time, morphs into something else & makes a sound, to try to scare you). Maybe we could start a separate topic for those, if they are popular enough?
Here's my new favorite:http://www.johnsadowski.com/big_spanish_castle.html
Stare at the dot in the center for 30 seconds or so, then (without moving your eyes) put your mouse somewhere in the image. As long as you don't move your eyes, your worn out cones will inject the correct colors into the black and white photo. And no, there is no "trick" to it, and to test it, try rolling your mouse onto the image BEFORE staring at it. It really IS a black and white photo, it is your eyes that are putting the correct colors there... Trippy, eh?