In physiology, a smile is a facial expression formed by flexing those muscles most notably near both ends of the mouth. The smile can also be found around the eyes (See 'Duchenne smile' below). 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 can be known as a grimace. There is much evidence that smiling is a normal reaction to certain stimuli as it occurs regardless of culture. Happiness is most often the motivating 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 also be a sign of fear. The study of smiles is a part of gelotology, psychology, and linguistics, comprising various theories of affect, humor, and laughter.
Biology is not the only academic discipline that interprets the smile. Those who study kinesics view the smile as an affect display. It can communicate feelings such as: love, happiness, pride, contempt, and embarrassment. More info: The Psychology of Human Smile, The Smile
A Duchenne smile contracts the zygomatic muscles of the cheek and eye, forming crow's feet. The crow's feet indicate that the smile is genuine and that the smiler is truly happy. It was discovered by and is named after Guillaume Duchenne.