Which way is she turning?

“Our visual system, if it has a choice, seems to prefer the view from above,” says Dr Niko Troje of Queen's University in Canada. “It’s a perceptual bias. It makes sense to assume that we are looking down onto objects that are located on the ground below us rather than floating in the air above us.”

According to a study performed by Dr Troje and his colleagues, viewing angle is all-important in the answer offered up by people's brains to the silhouette. As people are naturally predisposed to assume they are looking at things from above, they tend to assume they are and so decide that the lady is spinning clockwise.