incredible optical illusion

Incredibly, the squares marked A and B are the same shade of grey. I've reviewed the proof, I've read the explanation, and I am still amazed. The illusion was created by MIT Professor of Vision Science Edward Adelson. You can view more of his mind-warping creations here.

Bonus link: Pixie, from Nattyware. It is a tiny free program that gives you the hex, RGB, HTML, CMYK and HSV values of any color. It is a handy utility for web design and for verifying that A and B are the same shade of grey.