optical illusions

Check out this optical illusion, where a black and white image turns to color. Before you click the link, just so you're prepared, what you're going to do is stare at the black dot in the center, then wait 15 seconds for the negative to go back to black.

What's at work is called an afterimage, where the staring causes the [whatever color] receptor to get fatigued, so that when you switch to looking at plain white, which requires all the color receptors, [whatever color] is sending fewer messages to the brain, so we only see what's left, i.e. [opposite color] instead.

But when you do that, usually the afterimage is kinda fuzzy. What they've done here is add the black and white dots to help give the photo some definition, so all the afterimage has to do is give it a light wash of color. Also, during the 15-second negative phase, the dots are gray, to ensure that those receptors aren't fatigued when it switches back to the original image! Clever.

I almost wish you could select the duration of the negative; if you could stare longer, the afterimage would last longer.

One of my favorite (and most infuriating) illusions is The Spinning Dancer. This has happened to me twice: On first glance, I think she's spinning counter-clockwise; then, I close my eyes and mentally picture her spinning the other way; I open my eyes, and I'm able to see her spinning clockwise. But from then on, I seem incapable of seeing her going counter-clockwise again! I assure you, the animation remains exactly the same throughout.

The Wikipedia article on optical illusions has an extensive collection of more fun ones.