This ambiguous "Trick Mules Puzzle" is solved by the realization that the mule can have two different orientations. Here the same lines and contours have two interpretations, one horizontal and one vertical.
Over the years numerous advertising puzzles have been based on this puzzle.The earliest version of the puzzle used two dogs. and appeared in "The Magician's Own Book," published in 1857. In 1872 Sam Loyd, the great American puzzlist, adapted the concept, and produced "The Trick Mules Puzzle."