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"What was the price did Santa's sleigh cost? Zero, it was on the house."

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

This describes a joke-testing meeting with a firm that produces products for social events. Its catalogue features Christmas crackers.

The firm's owner grins, almost apologetically at the joke. But the joke has been selected and will appear in upcoming crackers.

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

The secret to a great Christmas cracker pun is not the identical as a good joke per se. It is all about the setting - in this case, the shared laughter of the Christmas dinner table with grandparents, children and possibly friends.

"The goal is for the joke to be something that brings the child together with the 80-year-old," she adds.

The Science Behind Shared Laughter

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

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

Communal laughter, she explains, helps forge and strengthen social connections between individuals.

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

"The people you converse with, and laugh with, it results in increased amounts of 'happy chemical' uptake," she continues.

These natural chemicals are the brain's "happy chemicals" and are released both to alleviate tension and discomfort and in response to enjoyable activities, such as chuckling with loved ones over a particularly terrible Christmas cracker joke.

"You're not just chuckling at a silly pun with a Christmas cracker," the expert says. "You are in fact doing a lot of the truly important work of building, preserving the connections you have with the people you love."

Which Happens In the Brain?

But what is truly happening inside the mind when we hear a joke?

An awful lot happens in reaction to humour, it transpires.

Using functional magnetic resonance imaging (fMRI), a type of neural imager which indicates which parts of the mind are working harder, researchers have been able to chart the regions that receive more blood flow.

The research involves imaging the brains of volunteer participants and then exposing them to a database of funny words, paired with either a non-emotional sound, or recorded laughter.

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

A gag activates not just the areas of the mind responsible for auditory processing and understanding language, but also brain areas associated with both preparation and starting movement and those linked to vision and memory.

Combine these elements together, and individuals hearing a pun have a complex set of neural reactions that support the laughter we hear.

The Infectious Nature of Laughter

Scientists discovered that when a humorous phrase is combined with chuckles there is a stronger response in the brain than the identical phrase when followed by a neutral sound.

"This was in parts of the mind that you would use to contort your face into a smile or a chuckle," the professor explains.

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

Amusement, according to the professor, can be infectious.

So what does this mean for the laughter found around a Christmas gathering?

"You laugh more when you are familiar with people," she notes, "and you laugh more when you are fond of them or love them."

When it comes to Christmas cracker jokes, she says, the positive factor is more probable to be triggered not by the joke in itself, but from the response to it.

"It's the laughter. The joke is the dreadful Christmas cracker pun, and it's just a reason to laugh together."

The Quest for the Perfect Festive Pun

Is it possible to find the ultimate joke?

Likely not, but that has not stopped researchers from trying to.

Years ago, a professor established a research search for the world's funniest joke.

More than tens of thousands of jokes later, with ratings lodged by 350,000 people globally, he has a clearer understanding than many as to what works and what fails.

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

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

The more "terrible" the joke, he states the more effective.

"The reason is that if no-one laughs – it's the gag's fault, not your own.

"What's interesting about the holiday cracker jokes is that none of us find them funny.

"That's a common experience around the gathering and I believe it's lovely."