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

This one-liner is met by moans that resonate through a storage facility in the capital.

This describes a humor-evaluation session with a firm that produces supplies for gatherings. Its catalogue includes festive crackers.

The firm's owner grins, nearly sheepishly at the gag. But the pun has been selected and will appear in upcoming crackers.

"You measure the joke by the number of moans and the intensity of the groans around the table," she says.

The secret to a great holiday cracker pun is not the identical as a stand-up joke per se. It is entirely about the context - in this case, the communal laughter of the Christmas dinner table with grandparents, children and possibly friends.

"You want the gag to be a thing that unites the eight-year-old in harmony with the grandparent," she adds.

The Neuroscience Behind Communal Laughter

Coming together to enjoy shared laughter is not only ancient, scientists say, it is probably to be older than humanity.

"So when you are chuckling with people around the holiday dinner you are engaging in what's very likely a truly ancient mammalian play vocalisation," says a professor.

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

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

"Those you talk to, and laugh with, it results in enhanced amounts of 'happy chemical' uptake," she adds.

These natural chemicals are the body's "happy chemicals" and are produced both to reduce tension and discomfort and in response to enjoyable activities, such as laughing with friends over a particularly awful Christmas cracker joke.

"You're not just chuckling at a foolish pun with a Christmas cracker," she says. "You are actually doing a lot of the really important work of building, preserving the connections you have with those you love."

What Happens In the Brain?

But what is actually taking place within the mind when we hear a joke?

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

Employing functional magnetic resonance imaging (fMRI), a kind of brain scanner which shows which parts of the mind are more active, scientists have been able to chart the regions that get more blood.

The research entails imaging the minds of volunteer subjects and then exposing them to a collection of humorous words, accompanied by either a non-emotional sound, or recorded chuckles.

"In the scanner we got a really fascinating pattern of neural activity," notes the neuroscientist.

A gag activates not just the areas of the mind in charge of hearing and interpreting language, but also brain areas involved in both preparation and initiating movement and those linked to sight and recall.

Put these elements as a whole, and individuals hearing a joke have a sophisticated set of neural reactions that support the amusement we experience.

The Contagious Power of Laughter

Researchers discovered that when a humorous phrase is combined with laughter there is a greater reaction in the mind than the same phrase when followed by a neutral sound.

"This was in areas of the brain that you would use to move your expression into a grin or a chuckle," she explains.

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

Laughter, says the expert, can be infectious.

So what does this mean for the chuckles heard around a Christmas table?

"You laugh more when you know people," she says, "and laughter increases further when you like them or love them."

When it comes to Christmas cracker jokes, she says, the feel-good factor is more probable to be caused not by the joke itself, but from the reaction to it.

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

The Quest for the Ideal Cracker Joke

Is it possible to discover the ultimate gag?

Likely not, but that has not stopped experts from attempting to.

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

Over tens of thousands of gags later, with ratings provided by hundreds of thousands of participants globally, he has a clearer understanding than most as to what works and what fails.

The perfect festive cracker joke needs to be brief, he explains.

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

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

"The reason is that if no-one finds it funny – it's the joke's shortcoming, not your own.

"What's interesting about the Christmas cracker jokes is that not one person find them funny.

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