In Alice’s Adventures in Wonderland, the titular heroine quaffs a potion that shrinks her down to the size of a doll, and eats a cake that makes her grow to gigantic proportions. Such magic doesn’t exist outside of Lewis Carroll’s imagination, but there are certainly ways of making people think that they have changed in size.

There’s nowhere in the world that’s better at creating such illusions than the lab of Henrik Ehrsson in Sweden’s Karolinska Institute. In a typical experiment, a volunteer is being stroked while wearing a virtual reality headset. She’s lyng down and looking at her feet, but she doesn’t see them. Instead, the headset shows her the legs of a mannequin lying next to her.

As she watches, Bjorn van der Hoort, one of Ehrsson’s former interns, uses two rods to stroke her leg, and the leg of the mannequin, at the same time. This simple trick creates an overwhelming feeling that the mannequin’s legs are her own. If the legs belong to a Barbie, she feels like she’s the ...