Smiles
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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