It didn’t work for me. They are close to the same color of grey on my monitor, but not the same. Maybe its my monitor. But still, amazing that they are so close.

My eyes frequently play tricks on me, part of my migraine issues, so I don’t trust my perception of things—but the squares can’t possibly be the same color! Even the one that changes color halfway through? Amazing.

This is a wonderful brightness illusion discovered by MIT vision scientist Ted Adelson. The light check in shadow is identical in luminance to the dark check outside the shadow (in other words, they are both identical grays), yet the perceived luminance (brightness) is different, because your visual system interprets the context of the scene (in shadow as being opposed to outside the shadow). Ted Adelson has a lot of very cool interactive brightness illusions on his website (and so does Dale Purves at Duke), or you can read about the science of them in some of my books on optical illusions. I have a pretty good selection of these illusions and others along with their explanations, as much as can be known about them at this time.