One day you might not have to ask someone to lend a helping hand–because you’ll have a third arm of your own. At least, that’s a possible application of a mental trick scientists performed on 154 healthy volunteers: These men and women were made to feel as if they had three arms.

To pull off this ruse, the researchers placed a prosthetic arm next to a volunteer’s two real arms, and they touched the subject’s right hand and the rubber hand in exactly the same place at the same time. Because the taps were synchronized, the volunteer’s brain was tricked into feeling them both. According to Science Daily:

“What happens then is that a conflict arises in the brain concerning which of the right hands belongs to the participant’s body,” says Arvid Guterstam, one of the scientists behind the study. “What one could expect is that only one of the hands is experienced as one’s own, presumably the real arm. But what we found, surprisingly, is that the brain solves this conflict by accepting both right hands as part of ...