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

This one-liner is met by groans that echo through a warehouse in London.

We're at a joke-testing session with a firm that produces supplies for gatherings. Its repertoire includes festive crackers.

The company's founder grins, almost apologetically at the gag. But the joke has made the cut and will appear in upcoming crackers.

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

The key to a great Christmas cracker joke is not the same as a stand-up joke per se. It is entirely about the setting - in this instance, the communal laughter of the Christmas dinner table with elders, children and potentially friends.

"The goal is for the gag to be something that brings the eight-year-old together with the 80-year-old," she states.

The Science Behind Shared Laughter

Gathering to enjoy shared amusement is not only nothing new, experts say, it is likely to be older than humanity.

"So when you are chuckling with others at the Christmas dinner you are dropping into what's almost certainly a really ancient mammalian social sound," says a neuroscience expert.

Shared laughter, she explains, aids in make and maintain social bonds between individuals.

Researchers have found that a lack of such interactions can seriously harm both psychological and bodily well-being.

"Those you talk to, and share laughter with, it leads to increased amounts of endorphin uptake," she adds.

These natural chemicals are the body's "happy chemicals" and are released both to reduce tension and discomfort and in response to enjoyable experiences, such as chuckling with friends over a truly awful festive cracker gag.

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

Which Occurs Inside the Brain?

But what is truly happening inside the brain when we listen to a gag?

A tremendous amount happens in response to humour, it transpires.

Employing functional magnetic resonance imaging (fMRI), a type of neural imager which shows which parts of the mind are more active, researchers have been able to chart the areas that get more blood.

The research involves scanning the brains of healthy participants and then exposing them to a collection of humorous phrases, paired with either a non-emotional sound, or recorded chuckles.

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

A gag activates not just the parts of the mind in charge of hearing and understanding speech, but also brain regions involved in both planning and initiating motion and those linked to sight and recall.

Put these elements together, and people hearing a joke have a complex series of brain reactions that support the amusement we hear.

The Infectious Nature of Laughter

Scientists discovered that when a humorous word is paired with laughter there is a greater reaction in the brain than the same phrase when accompanied by a neutral sound.

"This was in parts of the mind that you would employ to move your face into a grin or a laugh," she explains.

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

Amusement, according to the expert, can be contagious.

So what does this mean for the laughter found at a holiday gathering?

"People laugh harder when you know others," she notes, "and you laugh more when you like them or love them."

When it comes to festive cracker puns, she explains, the positive effect is more probable to be caused not by the joke itself, but from the response to it.

"The laughter is key. The gag is the dreadful holiday cracker joke, and it's just a reason to laugh as a group."

The Quest for the Ideal Cracker Joke

Is it possible to find the ultimate gag?

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

Years ago, a psychologist established a research search for the planet's most humorous gag.

Over tens of thousands of jokes later, with ratings lodged by hundreds of thousands of participants globally, he has a better idea than many as to what succeeds and what does not.

The ideal Christmas cracker joke must be brief, he explains.

"But they also be poor gags, puns that cause us to moan," he continues.

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

"The reason is that if nobody finds it funny – it's the gag's fault, not yours.

"The fascinating part about the holiday cracker puns is that not one person find them funny.

"It creates a shared experience at the table and I believe it's lovely."