Artists are known to have been playing with this phenomenon for hundreds of years. For example, take Leonardo da Vinci. "If you look at any walls spotted with various stains or with a mixture of different kinds of stones, if you are about to invent some scene you will be able to see in it a resemblance to various different landscapes adorned with mountains, rivers, rocks, trees, plains, wide valleys, and various groups of hills," he once said. "You will also be able to see diverse combats and figures in quick movement, and strange expressions of faces, and outlandish costumes, and an infinite number of things which you can then reduce into separate and well-conceived forms."

Or Dali. He would sometimes place an image on his canvas, and allow its form to suggest another that he would then paint within the first form, producing a double-image. This had often been done in the past as well. For instance, by the 16th-century Italian Mannerist, Arcimboldo. But in this regard, Dali was special. He would not stop. The great surrealist would repeat the process until he had produced triple and quadruple images. For him, the possibilities were endless.

Interestingly, researchers from the University of Helsinki in Finland have found that religious people and those who believed in paranormal phenomena saw faces where there weren't any more often than the non-religious and the skeptics. In one part of the test, the subjects had rated the face-likeness and emotional expression of the faces they saw. The set of supernatural believers was more likely than the skeptics to rate the illusory features as very face-like and emotional. I guess this explains at least a few of those UFO or Big Foot sightings that some people swear by.