Given a visual illusion with two interpretations, like the duck and the rabbit, our brain will switch between meanings. The phenomenon is an important evolutionary mechanism, say neurologists: "We need a trigger to prompt possible different interpretations so that we don't get stuck with a potentially incorrect interpretation of the world," says Ryota Kanai of University College London. "To find out which part of the brain might be involved, Kanai and colleagues asked 52 volunteers to watch a video of a revolving sphere and press a button when the rotation of the sphere appeared to change direction. Crucially, the sphere was not changing direction; it could simply be perceived to be rotating in either direction."