via www.scientificamerican.com
"Brightness and color can have powerful effects on perception. In this illusion created by vision scientist Edward H. Adelson of the Massachusetts Institute of Technology, squares A and B are the same shade of gray. (If you don’t believe it, print out this page, cut out the two squares and place them side by side.) Our brain does not perceive the true brightness and color of each square but instead determines the brightness and color of A and B by comparison with the squares surrounding them."
