Pareidolia: a psychological phenomenon involving a vague and random stimulus (often an image or sound) being perceived as significant.
In psychology, the Rorschach test is a series of images used to invoke pareidolia to delve into the psyche of the patient. In religion and superstition, a vague stimulus is believed to be divinely sent. Here is a news story of Mary in bird shit. Notice how the people react to a random stimulus.
No matter how much I look at this picture, it looks like a face. The sink looks a bit shocked or frightened.
Carl Sagan hypothesized that detecting faces is a hard wired evolutionary advantage. This allows people to use only minimal details to recognize faces from a distance and in poor visibility but can also lead them to interpret random images or patterns of light and shade as being faces.
In 2009 a study was done to show that objects incidentally perceived as faces evoke an early (165 ms) activation in the ventral fusiform cortex, at a time and location similar to that evoked by faces, whereas other common objects do not evoke such activation. This activation is similar to a slightly earlier peak at 130 ms seen for images of real faces. The authors suggest that face perception evoked by face-like objects is a relatively early process, and not a late cognitive reinterpretation phenomenon.
Which would explain why everyone sees the following simple line drawing as a face:
