Optical Illusions...

Here's a great collection of spectacular optical illusions.  In the one at right, the “yellow” tile is actually exactly the same color as the “brown” tile on top.  Don't believe it?  Make a mask from a piece of cardboard with two holes in it, and hold it over the image – you can see for yourself that they are the same color.

Perception is a very tricky thing, and the brain is far more involved that most people realize...