Hold your hand up in front of your face. It is patently obvious that the five-fingered thing in front of you is your hand, and the empty space next to it is not. But this ability to recognise your own body is more complicated than it first appears, and can be fooled through a surprisingly simple trick.

Henrik Ehrsson from the Karolinska Institute is a master of such illusion. When I visited his lab in 2011, he used little more than virtual reality headsets, mannequins and batons to convince me that I had left my body, shrunk to doll-size, and gained a third arm. Now, his team member Arvid Guterstam has devised a way of convincing people that they’ve got