This Spanish language exhibit directs you to look at a spinning spiral that appears to be advancing or receding for 30 seconds, and then look at a stationary object such as a wall. The wall will appear to be moving in the direction that's opposite to the spiral's motion. After you watch the spiral for 30 seconds, the receptors in your eyes that detect inward motion (or outward motion, depending on which way the spiral is spinning) adapt and stop responding. The receptors that that detect motion in the opposite direction briefly override them, so you see motion in that direction.
