You will probably have come across examples of pictographic ambiguity before, where a single drawing has more than one 'image' contained within it, depending on how you look at it. This picture, My wife and my mother-in-law, is a particularly good example, and was published in 1915 by the cartoonist W.E. Hill. Even experienced psychologists can sometimes find this hard. One clue - the chin of the young woman becomes the nose of the old lady.