Goblet illusion. This image is simultaneously a drawing of a goblet and two faces. When there are two competing interpretations of an ambiguous image the brain will often switch between them. So a viewer of this image, which has been around in some form since the 18th century, may see a white goblet for a while, but then change to seeing two black faces in profile. It is impossible to see both at the same time. It is also impossible to view this as abstract shapes - once the brain knows the form of a goblet and of a face in profile it must assign one of the two to its interpretation of this image.