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

This one-liner is met by moans that echo through a warehouse in the capital.

This describes a humor-evaluation session with a firm that makes products for gatherings. Its catalogue features festive crackers.

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

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

The key to a good Christmas cracker joke is not the same as a good joke per se. It is entirely about the context - in this instance, the communal amusement of the holiday dinner table with grandparents, children and potentially friends.

"The goal is for the gag to be something that unites the eight-year-old together with the grandparent," she adds.

The Science Of Communal Laughter

Gathering to enjoy shared amusement is not only nothing new, experts say, it is probably to be pre-human.

"Therefore when you are laughing with people around the Christmas table you are dropping into what's very likely a truly primordial mammal social sound," says a professor.

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

Researchers have found that a lack of these interactions can significantly damage both psychological and bodily health.

"Those you converse with, and laugh with, it leads to enhanced levels of 'happy chemical' release," she continues.

These natural chemicals are the brain's "feel-good compounds" and are produced both to alleviate tension and discomfort and in reaction to enjoyable activities, such as chuckling with friends over a truly terrible Christmas cracker joke.

"You're not just laughing at a silly pun with a Christmas cracker," she says. "You are in fact doing a lot of the really important work of building, preserving the connections you have with the people you care about."

Which Occurs In the Brain?

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

A tremendous amount occurs in response to humour, it turns out.

Employing brain scanning technology, a kind of neural imager which indicates which areas of the brain are more active, scientists have been able to map the areas that get more blood flow.

Testing involves imaging the brains of volunteer participants and then subjecting them to a database of humorous phrases, accompanied by either a non-emotional sound, or recorded chuckles.

"In the scanner we got a really interesting activation pattern of activation," notes the professor.

A gag activates not just the areas of the mind responsible for auditory processing and understanding language, but also brain areas involved in both preparation and starting movement and those linked to sight and recall.

Put these elements together, and individuals listening to a joke have a sophisticated series of neural reactions that underpin the laughter we hear.

The Contagious Nature of Chuckles

Scientists discovered that when a humorous word is combined with chuckles there is a stronger response in the brain than the same phrase when accompanied by a non-emotional sound.

"This was in areas of the mind that you would employ to contort your expression into a smile or a chuckle," she says.

It indicates we are not just responding to funny jokes, they are reacting to the amusement that follows them.

Amusement, says the professor, can be infectious.

So what does this imply for the laughter heard around a holiday table?

"You laugh more when you know others," she says, "and you laugh further when you are fond of them or care for them."

When it comes to festive cracker jokes, she explains, the positive effect is more probable to be triggered not by the gag itself, but from the reaction to it.

"It's the laughter. The gag is the dreadful Christmas cracker pun, and it's just a pretext to chuckle as a group."

The Search for the Perfect Cracker Joke

Will we ever discover the perfect gag?

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

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

More than 40,000 gags later, with scores lodged by hundreds of thousands of people around the world, he has a clearer understanding than most as to what succeeds and what does not.

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

"But they also need to be poor gags, jokes that cause us to groan," he continues.

The increasingly "awful" the joke, he says the more effective.

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

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

"It creates a shared experience at the gathering and I think it's wonderful."