Are the squares A & B different colors?
This checker board optical illusion shows the interesting way our brains perceive images.
This illusion focuses on the shadows on the board as a contrast to the lit up squares. It seems straightforward when we look at it, but if you cut the A and B squares out with a paint programme or such like, you will see that they are in fact exactly the same shade of grey. This can also be seen by printing the image out; it is only our brains that see the contrast.
The image is produced with permission from Professor Edward Adelson of M.I.T.