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

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

We're at a humor-evaluation meeting with a firm that makes products for social events. Its repertoire features Christmas crackers.

The company's owner grins, almost sheepishly at the gag. But the joke has made the cut and will feature in future crackers.

"You measure the gag by the number of moans and the loudness of the groans at the table," the founder explains.

The key to a great Christmas cracker joke is not the same as a good joke per se. It is entirely about the setting - in this case, the shared laughter of the holiday dinner table with grandparents, kids and potentially neighbours.

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

The Science Behind Shared Laughter

Gathering to enjoy shared laughter is not only nothing new, scientists argue, it is likely to be pre-human.

"So when you are laughing with people around the holiday table you are engaging in what's very likely a truly ancient mammalian social sound," explains a professor.

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

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

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

Endorphins are the body's "feel-good compounds" and are released both to reduce tension and discomfort and in response to pleasurable activities, such as chuckling with loved ones over a particularly terrible Christmas cracker gag.

"It's not simply chuckling at a foolish pun with a Christmas cracker," the expert says. "You are actually doing a lot of the truly important task of making, maintaining the social bonds you have with those you love."

Which Happens In the Brain?

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

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

Using brain scanning technology, a kind of neural imager which shows which areas of the brain are working harder, scientists have been able to chart the areas that receive more blood flow.

Testing entails imaging the minds of volunteer participants and then subjecting them to a database of funny words, paired with either a non-emotional sound, or recorded laughter.

"In the scanner we observed a very fascinating activation pattern of activation," says the neuroscientist.

A gag stimulates not just the parts of the mind in charge of hearing and interpreting language, but also neural areas associated with both planning and initiating movement and those linked to vision and recall.

Combine these elements as a whole, and individuals listening to a pun have a sophisticated series of brain reactions that support the laughter we experience.

The Contagious Nature of Chuckles

Scientists found that when a humorous phrase is paired with laughter there is a greater response in the brain than the identical phrase when accompanied by a neutral sound.

"This activation occurred in areas of the mind that you would employ to contort your expression into a grin or a chuckle," she explains.

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

Amusement, says the expert, can be contagious.

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

"You laugh harder when you are familiar with people," she notes, "and laughter increases further when you are fond of them or care for them."

When it comes to Christmas cracker puns, she explains, the positive factor is more likely to be triggered not by the joke in itself, but from the reaction to it.

"The laughter is key. The gag is the terrible holiday cracker pun, and it's just a reason to chuckle as a group."

The Search for the Perfect Festive Pun

Is it possible to discover the perfect gag?

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

In 2001, a psychologist set up a research project for the planet's funniest joke.

More than tens of thousands of jokes later, with scores lodged by 350,000 participants globally, he has a clearer understanding than most as to what succeeds and what fails.

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

"But they also be poor gags, jokes that make us moan," he continues.

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

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

"The fascinating part about the Christmas cracker jokes is that not one person considers them funny.

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