A smile is a facial expression formed primarily by flexing the muscles at the sides of the mouth.^[1] Some smiles include a contraction of the muscles at the corner of the eyes, an action known as a "Duchenne smile". Smiles performed without the eye contraction can be perceived as "fake".

Among humans, smiling is an expression denoting pleasure, sociability,happiness, or amusement. It is distinct from a similar but usually involuntary expression of anxiety known as a grimace. Although cross-cultural studies have shown that smiling is a means of communication throughout the world,^[2] there are large differences between different cultures, with some using smiles to convey confusion or embarrassment.

A smile seems to have a favorable influence upon others and makes one likable and more approachable.^[3] In the social context, smiling and laughterhave different functions in the order of sequence in social situations:

• Smiling is not a pre-laughing device and is a common pattern for paving the way to laughter;
• Smiling can be used as a response to laughter in the previous turn.^[4]

Smiling is a signaling system that evolved from a need to communicate information of many different forms. One of these is advertisement of sexual interest. Female smiles are appealing to heterosexual males,