You've probably heard of phantom limb syndrome. Because of odd wiring in the brain, a majority of amputees get the (frequently painful) sensation of having a limb, even after it's gone. But a team has shown that you don't have to be an amputee to have this feeling.
Neuroscientists at the Karolinska Institutet in Sweden created the illusion of a phantom limb in non-amputees like this. First, they had subjects sit down and place their arm behind a screen, so it was out of view. Next, behind the screen, the scientists tickled the unseen hand with a paintbrush. While they did that, the scientists waved a second paintbrush in front of the subjects, in full view. The two paintbrushes--the invisible one tickling the hand and the other just brushing the air in front of the subject--made the same movements.
The scientists discovered that the majority of subjects, within a minute, had the sensation of an invisible hand reaching out toward the paintbrush in mid-air. To test the sensation, researchers placed a knife in the air, where the the subjects reported feeling the third hand (eep). The subjects' sweat was measured as a way of testing stress, and when the knife was placed across the "invisible" hand, the subjects sweated more. As a control, if the knife was waved in front of the subjects without having the phantom hand induced, they didn't experience elevated stress.