You will need:
- a sheet of paper

Roll the paper into a tube. It should be at least large enough to stick 2 fingers into. Hold the tube up to your right eye, so that you are looking through it. Hold your left hand about 6 inches in front of your face, with your hand open, the palm facing towards you, and the base of your little finger against the side of the paper tube. With both eyes open, you should see a strange sight. You will see your hand in front of your face, with a hold through it. If you don't, try moving your hand a little farther away until it comes into focus.

What is happening? Normally, both of your eyes see the same thing, just from slightly different angles. Your brain combines these two slightly different views to let you see in three dimensions and judge distances. We have changed that. We are giving your brain two different images. One eye is seeing the palm of your left hand. The other eye is seeing the other side of the room, viewed through the tube. In combining the two images, your brain has to decide what is more important to see. Most of the view from your right eye is blocked by the dark sides of the tube, so you mostly see the view from your left eye. The one exception is the bright circle of image that your right eye sees through the tube. Since this is the one bright spot from your right eye, your brain pays extra attention to it. As your brain combines the two images, you wind up seeing your hand with a hole through it. If you want to play games with your brain, try watching television this way. Even better, have someone stick their finger into the end of the tube. That is a strange sight, indeed.