Your peepers are powerful, but they can also be fooled. Give this trick a spin and see what takes shape!
The Setup: Draw four black lines on an index card as shown, then poke a pushpin into the center of the back of the card.
The Action: Hold the pushpin with one hand. With the other, flick the card so that it spins rapidly. Do you see the double rings? What do you see when you change the shape, number, or color of the markings on your card?
What's Happening: As the card spins, the lines move before your eyes so fast that you can't actually see them. Instead, you see only the point on each line that is closest to the center of rotation (the pin). As these points move around that center, the resulting impression is of full concentric circles.