In physiology, a smile is a facial expression formed by flexing muscles most notably near both ends of the mouth, but also around the eyes. Among humans, it is customarily an expression of pleasure or amusement, but can also be an involuntary expression of anxiety. Studies have shown smiles to be a normal reaction to certain stimuli and occur regardless of culture, and is also not a reaction one learns, but is born with, as children blind from birth smile. 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.

Smiling not only changes a facial expression, but can also make the brain produce endorphins which reduces physical and emotional pain, and give a greater sense of well-being.

Types of smiles
Researchers have identified a number of different types of smiles.

1. The "Duchenne smile", after the researcher Guillaume Duchenne, is the most studied, and involves the movement of both the zygomatcus major muscle near the mouth and the orbicularis oculi muscle near the eyes. It is believed that the Duchenne smile is only produced as an involuntary response to genuine emotion, and is therefore what one could call the "genuine" smile.
2. The "Pan American smile", on the other hand, is the voluntary smile involving only the zygomatcus major muscle to show politeness; for example, by a flight attendant on the former airline of the same name. Considered 'insincere,' this type of smile has also been called the "Professional Smile" by David Foster Wallace in his comedic short story A Supposedly Fun Thing I'll Never Do Again (see link for relevant excerpt).
3. The "Chelsea smile", is a form of torture where one's mouth size is widened by considerably large cuts on the corners of the mouth.