What psychological phenomenon makes people see, for example, a face on the planet Mars?

Pareidolia is a common psychological phenomenon in which random stimulus is organized by the brain into a recognizable pattern (e.g., seeing faces in wallpaper patterns, or hearing musical beats in the sound of heavy machinery). Often used synonymously with the term "matrixing," pareidolia explains a large percentage of visual evidence mistakenly touted as definitive proof of the paranormal. In photographic evidence, it occurs in the tendency to pick shapes out of the blurry background or the random contrasts of light and shade (the face in a window, for example, that's merely the window pane reflecting the trees). In audio evidence, it occurs largely in the tendency to hear words and phrases in random sound; a good example of this is in Class C EVPs where words and phrases are discerned in the white noise, but without consensus as to what they are saying.