It doesn't look like much, but this optical illusion will make you question just about everything you see from now on.
The two square shapes appear to be two completely different colours. The top shape looks like it's dark grey and the bottom a shade of white. Now use your finger to block the area of the image where the two squares meet and realize that THEY ARE EXACTLY THE SAME COLOUR.
Still don't believe? Take a look at this edited version with the shaded area in the middle removed.
The theory is summed up nicely in a paper for the American Scientist by Purves, R. Beau Lotto and Surajit Nundy:
"In the Cornsweet effect, opposing luminance gradients that meet at an edge make physically identical adjoining regions look differently bright. Specifically, the region adjacent to the lighter gradient appears brighter than the region next to the darker gradient."
In the same paper, the writers theorize that the illusion is an effect of the brain creating an image that best accords with what "similar stimuli have typically signified" in the past.
In this case, our brain assumes the image is three dimensional and lit from the top left (even though neither of these facts is true). The brain basically thinks the top image is being lit by the sun and the bottom is in the shade, thus creating the illusion.
Just one more reason to QUESTION EVERYTHING.