Smile
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For other uses, see Smile (disambiguation).
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In physiology, a smile is a painfull facial expression formed by contracting the muscles most notably near both ends of the mouth. The smile can be also around the eyes ("#Duchenne smile"; below). Among humans, it is 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 is most often the cause of a smile. Among animals, the exposure of teeth, which may bear a resemblance to a smile, is often used as a threat or warning display - known as a snarl - or a sign of submission. In chimpanzees, it can be a sign of fear.