You must have seen these images, typically used for displaying optical illusions. For example, in the first image, do you see a duck or a rabbit? In the second image, do you see an old woman or young woman? Now National Geographic has reported on one of the oldest such illusions which comes from a Paleolithic cave in France.
In a particularly striking example, a small figurine has been given the details of a bison on one side and those of a mammoth on the other. The Paleolithic artist was clearly playing with the similar contours of the two animals and creating a single object that could be flipped to represent one species or the other.[World’s Oldest Optical Illusion Found? ]