There are many theories of humor which attempt to explain what humor is, what social function it serves, and what would be considered humorous.
It would be very difficult to explain humor to a hypothetical person who did not have a sense of humor already. In fact, to such a person humor would appear to be quite strange if not outright irrational behavior.
Among the prevailing types of theories that attempt to account for the existence of humor there are: psychological theories, the vast majority of which consider humor to be very healthy behavior; there are spiritual theories which may, for instance consider humor to be a "gift from God;" there are also theories that consider humor to be an unexplainable mystery, very much like a mystical experience.
Laughter is an audible expression or the appearance of happiness, or an inward feeling of joy (laughing on the inside).
It may ensue (as a physiological reaction) from jokes, tickling or other stimuli. It is in most cases a very pleasant sensation.
Laughter is found among various animals, as well as in humans.
Among the human species, it is a part of human behavior regulated by the brain, helping humans clarify their intentions in social interaction and providing an emotional context to conversations.
Laughter is used as a signal for being part of a group — it signals acceptance and positive interactions with others.
Laughter is sometimes seemingly contagious, and the laughter of one person can itself provoke laughter from others as a positive feedback. This may account in part for the popularity of laugh tracks in situation comedy television shows.
