This is an optical illusion by Edward H. Adelson who is a Professor of Vision Science at MIT.

Which square is darker – A or B?

The squares marked A and B are exactly the same shade of gray. If you don’t believe the answer, copy the image into your favorite graphics editor and check the colors. Or print the image, cut out the squares and hold them side by side. You’ll see they are identical.

Read more and see another video at the Quirkology Website