Hold out the index finger from your right hand, and rub it a few times with the index finger, and thumb of your left hand.
Then, hold a pencil in the right hand, so that it extends under your index finger. Rub your finger again, but this time, touch the pencil, instead of to the bottom of your finger. Your finger should begin to tingle.
The first time you touch your finger, nerves from the top, and the bottom of the right hand index finger, and nerves from the thumb, and index finger of the left hand, send messages to the brain that they are touching. The second time, the thumb, and index finger, of the left hand, send the signals, but one set of signals will be missing from the right hand, tricking the brain into reacting as if the finger is asleep.