(15 minutes or less)
Cut out the copy of the pattern disk provided as a separate page, here and mount it on cardboard with adhesive or tape.
Attach the disk to some form of rotator. (The adhesive-backed Velcro™ provides a convenient way to mount the disk to a drill or similar device.)
For a low-tech method of rotating the disk, push the pushpin through the center of the pattern into the eraser of a pencil. Spin the disk by hand as you hold the pencil.
(5 minutes or more)
Rotate the disk slowly (1 or 2 seconds per revolution) and stare at its center for about 15 seconds. Now look at the palm of your hand. Notice that your palm seems to be turning. Your palm will turn in the opposite direction from the way the disk was turning.
Mechanisms in your eye and brain detect motion in various directions. For example, regions of your brain fire nerve impulses when your eye forms images that are rotating in a clockwise direction. Other regions respond to counterclockwise rotation. When something is stationary, both of these motion detectors still fire, but their firing rates are equal. The two signals balance each other out, and you see no motion.
As you stare at the spinning disk, the set of motion detectors that respond to its rotation adapts to the motion of the pattern. These motion detectors start out firing rapidly, and then slowly decrease their firing rate. When you look away from the rotating pattern and stare at a stationary object (such as your palm), the motion detectors that have been firing fire less rapidly than the ones that have not been stimulated. As a result, you see motion in the opposite direction.
You also have sets of motion detectors that respond to upward and downward motion. Adaptation of these upward and downward motion detectors causes the version of the waterfall effect that you notice when you watch a waterfall.