Would you believe that the square marked A and the square marked B are actually the exact same shade of grey? To prove it, you can click and drag the blue mask over the squares.
This illusion was created by Edward H. Adelson, a professor of vision science at MIT, who explains that the visual system compensates for the perceived shadow over the B square, perceiving it as lighter than it really is. The fact that the green cylinder is visible, supposedly casting the shadow, adds to the effect. The visual system ignores the gradual changes in shade near the fuzzy edges of the shadow. In addition, the contrast of the light squares surrounding the A square make it appear darker, and the dark squares surrounding the B square make it appear lighter.