Wearing an ordinary pair of sunglasses with one lens removed, and standing a few feet away, a person watches the pendulum swing back and forth. The orange tip of the pendulum appears to be moving in a circle, towards and away from the person, rather than just back and forth. Switching glasses, with the opposite lens removed, the circle reverses direction.

What's happening is that one eye is receiving less light, so the image is slightly delayed relative to the other eye. During that short time delay, the pendulum moves, so one eye receives an image that is slightly shifted in position. The brain interprets this relative shift as depth, so the pendulum appears to move forward and backward. The greatest shift happens when the pendulum is moving the fastest, at the middle of the swing, so that's when the pendulum appears to have the greatest shift forward or backward, and the motion appears to be circular.

• sunglasses
• a ruler or other flat piece of wood or plastic
• nut, bolt, washers, screws
• wood post (13 x 2 x 3/4 inch)
• wood platform (8.5 x 11 x 3/4 inch)
• table clamps (optional)

Drill a hole through the post for the bolt. Mount the post vertically on the platform using screws.

Wrap orange or yellow duct tape around the end of the ruler, or paint it a bright color. If there isn't already a hole in the end of the ruler, drill one.

Put a washer on the bolt, then thread the bolt through the hole in the ruler (make sure the hole is large enough so that the ruler can turn freely around the bolt), add another washer, thread the bolt through the hole in the post, and secure with a nut on the other side. Don't screw it too tightly, you want the ruler to turn freely.

Pop out the left lens from half of the pairs of sunglasses, and pop out the right lens from the rest.

When displaying the pendulum, optionally clamp the platform to a table to make it more stable.