#13: The Cornsweet Illusion

Two identical shades, divided by a line.

Put your finger in the middle and watch. Two identical gray areas are split by a sharp edge with light on one side and dark on the other. Cover the edge with your finger, and suddenly the two sides become the same color. Why it works: Your brain uses the contrast at the edge to guess the shading of entire areas, making one side look lighter and the other darker.