In bursts, with the mouth wide open, like a fool, like a hunchback, like a whale. In puffs, in explosions, stifling, snorting, rattling, with tears... It is like a tide, laugh is irresistible.

The human laugh is executed by muscle designed for it. We may burst with all our bodies, but the face has 12 specialized muscles for laughing. In laughing the muscles called risorius (which dilates the nostrils - in Latin "ridere" means to laugh) and zygomaticus (which raises the upper lip) are very important.

Our heartbeat increases and the arteries dilate. Our abdominal muscles contract with a terrible force. Meanwhile, the hypothalamus, the control center of our brain, transmits to the other brain areas feel-good chemicals called catecholamines (like dopamine, adrenaline, and noradrenaline) and endorphins, analgesic hormones usually released after strenuous exercise, excitement, and orgasm.

The laugh is mutual and empathetic. We understand its significance and we start laughing because the others around us are laughing.

At the age of three months, human babies note that any of their smiles provoke a pleasant reaction from their mothers: they smile back, speak, and tickle them. Because of the laugh game, the relation mother-baby intertwines, turning increasingly stronger. The laugh is a method that makes any person to feel s/he is not lonely, that her/his parents, brothers, sisters or friends take care of her/him and protect her/him. Laughing is maybe the best weapon humans can use against fear.

Researchers believe that the human laugh originated in power displays of the monkeys. In species like baboons, macaques, or apes, the males have huge canine teeth, compared to the females. The dominant male yawns from time to time, displaying his huge fangs, a gesture meant to cut the courage of the other males to confront him. Something like: "Look how strong I am". When two male monkeys confront each other, they show each other their canines, as if saying "I'm not afraid of anything!"

Along the evolution, this turned into a bluff, as rivals did not use any longer their smaller canines in fights (but apes, like orangutans, do and the wounds are often deadly). Humans could show one another their teeth without posing any danger: they laughed at each other. It is just a theory. In fact, psychologists consider laughing a form of aggression.

Humor has turned in time in a human cultural trait: jokes and the sense of humor are very specific to your own culture. One example is the British black humor.

And take into account that some animals laugh too.