You’ve seen this optical illusion in movies but it happens when you’re watching in person too. It’s a familiar scene to most anyone with a television: The wheels of a forward-moving vehicle will appear at first to spin in one direction. The car puts on some speed and, as you would expect, its wheels rotate faster. But then, the something goes screwy. At a certain point, the spin of the wheels appears to slow, slow, slow. Then, ever so briefly, it stops. When it resumes, the spin is in the opposite direction. By appearance, the car should be moving backward and yet, forward it rolls.

This phenomenon is known as the “wagon-wheel” effect. If, like most people, you’re accustomed to seeing the wagon-wheel effect in movies or TV, its explanation is fairly straightforward: Cameras record footage not continuously, but by capturing a series of images in quick succession, at a specified “frame rate.” With many movie cameras, that rate is 24 frames per second. When the frequency of a wheel’s spin matches the frame rate of the camera recording it (say, 24 revolutions per second), each of the wheel’s spokes completes a full revolution every 1/24 seconds, such that it ends up in the same position every time the camera captures a frame.

The wagon wheel effect, as seen on film and television, is easily explained. Less clear, however, is why people experience the wagon-wheel effect not through a screen or by virtue of strobe lighting, but out in the real world, under constant lighting conditions. There are presently two competing hypotheses that account for this effect.