On Sunday the whole world was laughing -- literally.
The first Sunday in May marks International Laughter Day, a holiday established in 1998. Now, eight years later, cities all around the world have planned their own special celebrations to mark the day of smiles.
No one doubts that laughing makes us feel good, but why do we laugh? After all, it's not exactly a trait we share with other animal species.
"It's a signal that it's safe to relax," said psychologist Steve Wilson, who is co-founder of the World Laughter Tour.
Laughter activates reward centers in the brain, Wilson said, causing a pleasant release of tension within the body. The ability to laugh is innate, and we're hardwired to laugh about a month or so after birth.
Anthropologists believe that laughter dates back millions of years -- we were laughing long before we were verbally communicating. Whether you speak Chinese, English or Swahili, a laugh or smile has virtually the same meaning all over the world, researchers said.
"At it's base, humor is connection between people that goes far beyond language," said Dr. Clifford Kuhn, a psychiatrist at the University of Louisiana Medical Center.
Is It Contagious?
Yes. And most people have experienced what Americans call a case of the giggles.
"We laugh 30 times as much when we're with other people than we do when we are alone," said Robert R. Provine, a professor of psychology and neuroscience at the University of Maryland Baltimore County. "Laughing is not a solo activity."
Unbelievably, an epidemic of contagious laughter struck the African country of Tanzania in the 1960s. A school temporarily closed down after three schoolgirls set off about two-thirds of the other students into uncontrollable hysterical laughter.
While there were no real consequences, it just goes to show the contagious element of laughing.
A Laugh a Day Keeps the Doctor Away?
Many studies have found that laughter does a number of positive physical things such as reduce stress, lower blood pressure, boost the immune system and protect the heart.
But Provine, who wrote "Laughter: A Scientific Investigation," remains skeptical.
"There's no doubt that laugher is a good thing," he said. ... "But I think there are a lot of presumed medical effects that have been created from social circumstances and by the media."
He gives an example that comes from a Florida study that found comedy had a mild effect on pain. But then so does dramatic entertainment.
"Distraction can be a very compelling factor," he said.
Regardless, educators, doctors, community leaders and employers have created a new market -- the business of making people laugh.
Kuhn said he even prescribes his patents the "ha ha ha" prescription.
Provine, although more conservative in his views about the health effects of laugher, said "anytime you can make people laugh it's a good thing."
Can Laughter Kill?
"There's a lot of folklore on this and not a lot of research," Provine said. "In rare circumstances laughing can cause narcolepsy or induce asthma attacks. ... But it is unlikely that it will ever kill you. That sharp pain you feel in your side when you laugh too hard is just a result of your diaphragm muscle contracting."
In ancient times however, tickling was used as a form of torture -- goats would lick salt off the bottom of a person's foot.
Even the great thinkers of the time didn't know what to make of laughter. The philosopher Plato wrote a book proposing a laughter ban.
"Can you imagine today if someone in Congress tried to pass a laughter ban?" Provine joked.
Can Laughter Burn Calories?
If you thought that laughter could replace your daily exercise regime, keep dreaming.
Researchers at Vanderbilt University found that laughter does burn calories -- but you would have to laugh for about 15 minutes a day, 365 days a year to lose just four pounds.
"Fifteen minutes a day is a lot of laughing," Wilson said. "It's very unlikely that most people will laugh that much in a day."