It 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, 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. Many studies indicate that smiling is an innate reaction, as children blind from birth, and even human fetuses smile

however feral children generally do not smile, perhaps lending evidence to the contrary, perhaps indicating the profound psychological disturbance that results in a member of a social species brought up with no elders or peers. 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.