A Smile

A smile is a facial expression formed by flexing the muscles near both ends of the mouth. The smile can also be found around the eyes. 

Among humans, it is customarily an expression denoting pleasure, happiness, or amusement, but can also be an involuntary expression of anxiety, in which case it is known as a grimace.

Cross-cultural studies have shown that smiling is a means of communicating emotions throughout the world. But there are large differences between different cultures.  A smile can also be spontaneous or artificial (when people feel obliged to smile).

Happiness is most often the motivating cause of a smile. 

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(Thanks Wikipedia)