#12: The Checker Shadow Illusion

Believe it or not, squares A and B are the same shade of gray.

This famous illusion by Edward Adelson shows a checkerboard with a shadow cast over it. A square labeled 'A' in the light appears much darker than a square labeled 'B' in the shadow, but they are physically the same color. Why it works: Your brain accounts for shadows when interpreting colors, adjusting your perception to maintain consistency in lighting.