b belly laugh life.

Try to make yourself laugh. You'll probably only get a tight-lipped chuckle. Why? "Laughter is almost always something you do with other people," says professor Robert Provine of the University of Maryland.

Your brain automatically triggers laughter during play, or positive social interaction. And the whole body reacts (see diagram, right). "It's difficult to laugh on command," says Provine.

Scientists are still unsure what triggers a giggling fit. But they do know most laughter isn't about jokes. Babies first laugh about three or four months after birth, way before they can understand a joke.

Other animals also laugh. "There are dramatic similarities between play-induced chirping in rots and play laughter in children," says Jaak Panksepp of Bowling Green University in Ohio. When rats are tickled behind the neck, they let out a series of chirps higher than humans can hear.

Some hesitate to call the rats' noise laughter. But most scientists agree that chimps and apes can laugh. "No other animal makes the classic human ha-ha," says Provine. "But our closest primate relatives do indeed laugh. The sound is more like pant-pant." In apes, laughter is a signal to other animals that they're not being aggressive, only playing.

Provine believes we guffaw for the same reason. The sound and body responses that we associate with human laughter may have evolved from a more basic chuckle in our ancestors. "During evolutionary history, pant-pant became ha-ha," says Provine. We produce the sound as a survival instinct (inborn ability) to communicate a social bond. No joke.

HAVE A LAUGH

Scientists still don't know what happens to the body during a belly laugh, but it revolves the whole body. Next time you laugh hard, record your pulse (beats per minute), wait 15 minutes, then take it again. Are your observations consistent with the diagram, below?

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