Pareidolia (// parr-i-DOH-lee-ə) is a psychological phenomenon involving a vague and random stimulus (often an image or sound) being perceived as significant. Common examples include seeing images of animals or faces in clouds, the man in the moon or the Moon rabbit, and hearing hidden messages on records when played in reverse.
The word comes from the Greek words para (παρά, "beside, alongside, instead") in this context meaning something faulty, wrong, instead of; and the noun eidōlon (εἴδωλον "image, form, shape") the diminutive of eidos. Pareidolia is a type of apophenia, seeing patterns in random data.
In his notebooks, Leonardo da Vinci wrote of pareidolia as a device for painters, writing "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. You will also be able to see divers 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."
There have been many instances of perceptions of religious imagery and themes, especially the faces of religious figures, in ordinary phenomena. Many involve images of Jesus, the Virgin Mary, the word Allah, or other religious phenomena: in September 2007 in Singapore, for example, a callus on a tree resembled a monkey, leading believers to pay homage to the "Monkey god" (either Sun Wukong or Hanuman) in the monkey tree phenomenon.
Publicity surrounding sightings of religious figures and other surprising images in ordinary objects has spawned a market for such items on online auctions like eBay. One famous instance was a grilled cheese sandwich with the Virgin Mary's face.
Various European ancient divination practices involve the interpretation of shadows cast by objects. For example, in molybdomancy, a random shape produced by pouring molten tin into cold water is interpreted by the shadow it casts in candlelight.
From the late 1970s through the early 1980s, Japanese researcher Chonosuke Okamura self-published a famous series of reports titled "Original Report of the Okamura Fossil Laboratory" in which he described tiny inclusions in polished limestone from the Silurian period (425 mya) as being preserved fossil remains of tiny humans, gorillas, dogs, dragons, dinosaurs, and other organisms, all of them only millimeters long, leading him to claim "There have been no changes in the bodies of mankind since the Silurian period... except for a growth in stature from 3.5 mm to 1,700 mm." Okamura's research earned him an Ig Nobel Prize (a parody of the Nobel Prizes) in biodiversity. See List of Ig Nobel Prize winners (1996).
The Rorschach inkblot test uses pareidolia in an attempt to gain insight into a person's mental state. The Rorschach is a projective test, as it intentionally elicits the thoughts or feelings of respondents which are "projected" onto the ambiguous inkblot images. Projection in this instance is a form of "directed pareidolia".
Electronic voice phenomenon
Carl Sagan hypothesized that as a survival technique, human beings are "hard-wired" from birth to identify the human face. 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. The evolutionary advantages of being able to discern friend from foe with split-second accuracy are numerous; people who accidentally identify an enemy as a friend could face deadly consequences. 
A 2009 magnetoencephalography study found 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. An fMRI study in 2011 similarly showed that repeated presentation of novel visual shapes that were interpreted as meaningful led to decreased fMRI responses for real objects. These result indicate that interpretation of ambiguous stimuli depends on similar processes as those elicited for known objects.
These studies help to explain why people identify a few circles and a line as a "face" so quickly and without hesitation. Cognitive processes are activated by the "face-like" object, which alert the observer to both the emotional state and identity of the subject – even before the conscious mind begins to process – or even receive – the information. The "stick figure face", despite its simplicity, conveys mood information (in this case, disappointment or mild unhappiness). It would be just as simple to draw a stick figure face that would be perceived (by most people) as hostile and aggressive. This robust and subtle capability is hypothesized to be the result of eons of natural selection favoring people most able to quickly identify the mental state, for example, of threatening people, thus providing the individual an opportunity to flee or attack preemptively. In other words, processing this information subcortically (and therefore subconsciously) – before it is passed on to the rest of the brain for detailed processing – accelerates judgment and decision making when alacrity is paramount. This ability, though highly specialized for the processing and recognition of human emotions, also functions to determine the demeanor of wildlife.
Combined with Apophenia (identifying meaningful patterns in meaningless randomness) and hierophany (a manifestation of the sacred), pareidolia may have helped early societies organize chaos and make the world intelligible.
The Romanian Sphinx in Bucegi Mountains
Human face on Pedra da Gavea in Rio de Janeiro, Brazil, known as "Head of the Emperor"
Smiley face in Galle Crater on Mars
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