Look at the images below. The first is a vase, right? And the second? Would you believe it is two people looking at each other?
This is Rubin's famous vase, an optical illusion developed by the Danish psychologist Edgar Rubin. Illusion presents the observer with a mental choice between two valid interpretations: the silhouette of a vase or the profile of two human faces. Usually, the observer perceives only one of them and only after some time ends up realizing the second.