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"How much did Father Christmas's sleigh cost? Nothing, it was on the house."

This quip is greeted with groans that resonate through a warehouse in the capital.

This describes a humor-evaluation session with a company that makes supplies for gatherings. Its repertoire includes Christmas crackers.

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

"The success is gauged by the joke by the volume of moans and the loudness of the groans around the table," the founder explains.

The secret to a great holiday cracker joke is not the identical as a stand-up gag per se. It is all about the setting - in this instance, the shared laughter of the Christmas dinner table with elders, kids and possibly neighbours.

"The goal is for the joke to be a thing that brings the child in harmony with the 80-year-old," she adds.

The Science Behind Communal Laughter

Coming together to enjoy shared amusement is not only ancient, scientists argue, it is likely to be pre-human.

"So when you are laughing with people at the Christmas dinner you are dropping into what's very likely a really primordial mammalian play vocalisation," explains a neuroscience expert.

Communal amusement, she explains, aids in forge and strengthen social connections between people.

Researchers have found that a lack of such social exchanges can significantly harm mental and physical health.

"Those you talk to, and share laughter with, it leads to enhanced levels of endorphin release," she adds.

These natural chemicals are the body's "happy chemicals" and are released both to alleviate tension and discomfort and in reaction to enjoyable activities, such as chuckling with loved ones over a truly terrible festive cracker gag.

"You're not just chuckling at a silly joke with a holiday cracker," the expert says. "You are actually doing a lot of the truly important task of making, maintaining the connections you have with the people you care about."

Which Happens In the Brain?

But what is actually happening inside the brain when we hear a joke?

An awful lot occurs in response to humour, it turns out.

Using functional magnetic resonance imaging (fMRI), a kind of brain scanner which indicates which parts of the brain are working harder, scientists have been able to map the areas that receive more blood flow.

The research entails imaging the minds of healthy participants and then subjecting them to a database of humorous phrases, accompanied by either a non-emotional sound, or recorded laughter.

"In the scanner we observed a really interesting activation pattern of neural activity," says the professor.

A gag stimulates not just the parts of the mind responsible for hearing and understanding language, but also neural regions associated with both planning and starting movement and those involved in vision and recall.

Put these elements together, and people listening to a pun have a sophisticated set of neural reactions that support the amusement we hear.

The Infectious Power of Laughter

Researchers found that when a funny phrase is combined with laughter there is a greater reaction in the brain than the same word when followed by a non-emotional sound.

"This was in areas of the brain that you would employ to contort your face into a smile or a chuckle," she explains.

It indicates people are not just reacting to humorous jokes, they are responding to the amusement that accompanies them.

Amusement, according to the professor, can be infectious.

So what does this mean for the chuckles heard at a holiday table?

"People laugh more when you know others," she notes, "and you laugh more when you are fond of them or love them."

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

"It's the laughter. The gag is the dreadful holiday cracker pun, and it's just a reason to chuckle together."

The Search for the Perfect Cracker Joke

Is it possible to find the perfect joke?

Probably not, but that has not prevented researchers from attempting to.

In 2001, a psychologist set up a scientific search for the world's funniest joke.

More than 40,000 jokes later, with ratings lodged by hundreds of thousands of people around the world, he has a better idea than many as to what works and what fails.

The ideal festive cracker joke must be short, he says.

"They must also be poor jokes, jokes that make us groan," he continues.

The increasingly "awful" the gag, he states the more effective.

"The reason is that if no-one laughs – it's the joke's fault, not yours.

"What's interesting about the Christmas cracker puns is that none of us find them humorous.

"It creates a common experience around the table and I believe it's wonderful."