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"How much did Santa's sled cost? Zero, it was on the house."

This quip is greeted with moans that echo through a warehouse in the capital.

We're at a joke-testing session with a firm that makes products for social events. Its catalogue includes festive crackers.

The company's founder grins, nearly apologetically at the gag. But the pun has made the cut and will feature in future crackers.

"You measure the joke by the number of groans and the loudness of the groans at the table," she says.

The secret to a good holiday cracker joke is not the same as a stand-up joke per se. It is all about the setting - in this case, the shared laughter of the holiday dinner table with elders, children and possibly neighbours.

"You want the joke to be something that unites the eight-year-old together with the 80-year-old," she states.

The Neuroscience Behind Shared Laughter

Coming together to experience communal amusement is not only ancient, scientists say, it is likely to be pre-human.

"So when you are laughing with others around the holiday table you are dropping into what's almost certainly a truly ancient mammal play vocalisation," says a professor.

Shared amusement, she says, helps forge and strengthen social connections between individuals.

Scientists have discovered that a absence of such interactions can seriously harm mental and physical well-being.

"The people you talk to, and share laughter with, it leads to increased amounts of 'happy chemical' uptake," she adds.

These natural chemicals are the body's "feel-good compounds" and are produced both to alleviate stress and pain and in response to enjoyable experiences, such as chuckling with loved ones over a truly terrible festive cracker gag.

"It's not simply laughing at a silly joke with a holiday cracker," she says. "You are in fact performing a lot of the really important task of building, preserving the connections you have with the people you love."

Which Happens In the Mind?

But what is actually taking place within the brain when we listen to a joke?

An awful lot occurs in reaction to comedy, it turns out.

Employing brain scanning technology, a kind of brain scanner which indicates which parts of the mind are working harder, researchers have been able to chart the regions that get more blood flow.

The research involves imaging the brains of healthy subjects and then subjecting them to a database of humorous phrases, accompanied by either a neutral sound, or recorded laughter.

"During the study we got a really interesting activation pattern of activation," notes the professor.

A joke stimulates not just the parts of the mind in charge of hearing and interpreting language, but also neural regions involved in both planning and starting motion and those involved in sight and recall.

Put all of this together, and people hearing a joke have a sophisticated series of brain responses that underpin the laughter we experience.

The Infectious Power of Chuckles

Researchers discovered that when a funny phrase is combined with laughter there is a stronger response in the mind than the same word when accompanied by a neutral sound.

"This activation occurred in parts of the mind that you would use to move your face into a smile or a chuckle," she says.

It indicates we are not just reacting to funny words, they are reacting to the laughter that accompanies them.

Laughter, according to the expert, can be infectious.

So what does this imply for the chuckles heard at a Christmas table?

"People laugh harder when you know others," she says, "and you laugh further when you like them or care for them."

When it comes to Christmas cracker jokes, she explains, the feel-good effect is more probable to be triggered not by the gag itself, but from the response to it.

"The laughter is key. The joke is the dreadful Christmas cracker pun, and it's just a pretext to chuckle together."

The Quest for the Perfect Festive Pun

Is it possible to discover the ultimate gag?

Likely not, but that has not prevented experts from trying to.

In 2001, a professor set up a scientific project for the world's funniest gag.

Over tens of thousands of jokes later, with ratings provided by hundreds of thousands of participants around the world, he has a better idea than most as to what succeeds and what does not.

The ideal festive cracker joke needs to be brief, he says.

"They must also be poor gags, puns that cause us to groan," he adds.

The more "awful" the joke, he says the better.

"This is because if no-one laughs – it's the gag's fault, not yours.

"The fascinating part about the holiday cracker jokes is that none of us considers them humorous.

"That's a common moment around the gathering and I think it's lovely."