Have you ever wanted to make your friends believe that there is an invisible animal crawling on them? It's easy. Just build up some coordination, make some very long-suffering friends, and create the Cutaneous Rabbit Illusion on someone's arm.
The cutaneous rabbit illusion is proof that we don't know our bodies, or reality, or both, as well as we think we do. The procedure is simple. Tap, rapidly, two different points of your body. The best spots are up near your elbow, and then down near your wrist. Keep tapping, elbow, wrist, elbow, wrist, and see what you feel. Soon enough, you should feel as if there was an invisible hopping animal moving from one spot to the other.
The illusion works in part because our brain, not our nerves, determines how we experience the world. The brain isn't bad at interpreting stimuli, but it has a lot to deal with, and it leans on its past experiences. Those experiences don't include rapid syncopated tapping on different sites from two difference sources, but they do include slow movement from one spot to another. Given a completely strange sensation, the brain will try to interpret it as a familiar event.
The brain is helped along in this illusion by the fact that not all of your body is the familiar territory you think it is. We're very good at understanding what's happening to sensitive, frequently-touched parts of our body, like our fingers or our face. Not all of the body is mapped out quite that clearly. The territory between the elbow and the wrist isn't well-defined because it's never had to be especially sensitive. Though we know something is touching us in that approximate area, we don't know exactly where, and so it's easier to make up a location. When we think about the illusion this way, the Cutaneous Rabbit Illusion is a kind of test of which parts of our body are not well mapped. Just tap the different parts of your body, and see which body parts make the "rabbit" appear.
[Via The Cutaneous Rabbit]