Neuroscientist Greg Gage demonstrates how to control someone else's arm with your mind in a creepy TED demonstration. By reading the electrical signals that the brain sends through the spinal cord and out into nerves that run through the arms, Gage hijacks the impulses and makes a volunteer's arm jump and twitch. Gage's company, Backyard Brains, makes cheap and easy-to-use neuroscience equipment for people to try this at home, but warns of the unpleasant side effects of losing your free will.