In physiology, a smile is a facial expression formed by flexing the muscles most notably near both ends of the mouth. The smile can be also around the eyes. Among humans, it's customarily an expression of pleasure, happiness, or amusement, but can also be an involuntary expression of anxiety, in which case it can be known as a grimace. There is much evidence that smiling is a normal reaction to certain stimuli and occurs regardless of culture. Happiness most often causes the smile to occur, though. Among animals, the exposure of teeth, which may bear a resemblance to a smile, are often used as a threat - known as a snarl - or a sign of submission. In chimpanzees, it can be a sign of fear.
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