A cool optical illusion!
The boxes marked A and B are of the same shade of gray!!
Don't believe me? Test it by yourself! Cover all the squares except A and B with your hand and now look.. If that's not convincing, open the picture in a photo editing software and check for the colour values!
How it works?
From MIT website .. http://www-bcs.mit.edu/people/adelson/checkershadow_illusion.html