A trick of shadow makes two identical greys look like different colors

In Edward Adelson's 1995 checker-shadow illusion, two squares on a checkerboard are printed in the exact same shade of grey, but one sits inside a cast shadow. Your visual system automatically discounts the shadow when judging color, so the shadowed square reads as far lighter than it actually is. Painting a solid bridge between the two squares is the only way to convince most viewers they're identical.

— Edward H. Adelson, Checker Shadow Illusion — MIT Perceptual Science Group, 1995