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"What was the price did Santa's sled cost? Nothing, it was on the house."

This quip is met by groans that resonate through a storage facility in London.

This describes a joke-testing session with a company that produces products for gatherings. Its repertoire features Christmas crackers.

The firm's owner smiles, almost apologetically at the gag. But the joke has made the cut and will feature in future crackers.

"You measure the joke by the volume of groans and the intensity of the groans around the table," the founder explains.

The secret to a great Christmas cracker pun is not the same as a good gag in itself. It is entirely about the setting - in this case, the shared amusement of the holiday meal with elders, kids and potentially neighbours.

"You want the joke to be something that unites the eight-year-old together with the grandparent," she states.

The Science Behind Shared Laughter

Coming together to enjoy communal laughter is not only ancient, experts argue, it is likely to be older than humanity.

"Therefore when you are laughing with people around the Christmas dinner you are engaging in what's almost certainly a truly primordial mammal play sound," explains a professor.

Shared amusement, she explains, helps make and maintain social bonds between people.

Scientists have found that a absence of these interactions can seriously damage both psychological and bodily well-being.

"Those you converse with, and share laughter with, it leads to increased levels of 'happy chemical' uptake," she continues.

Endorphins are the brain's "feel-good compounds" and are produced both to reduce tension and discomfort and in reaction to enjoyable activities, such as chuckling with loved ones over a truly terrible Christmas cracker joke.

"You're not just laughing at a foolish joke with a holiday cracker," she says. "You are in fact doing a lot of the really vital task of building, preserving the social bonds you have with those you care about."

What Occurs In the Mind?

But what is actually happening inside the brain when we hear a gag?

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

Using functional magnetic resonance imaging (fMRI), a kind of neural imager which indicates which areas of the mind are working harder, scientists have been able to map the areas that receive more blood flow.

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

"In the scanner we observed a very fascinating pattern of neural activity," notes the neuroscientist.

A gag stimulates not just the parts of the brain responsible for hearing and understanding speech, but also neural regions involved in both planning and initiating motion and those linked to vision and recall.

Put all of this together, and people hearing a pun have a sophisticated series of brain reactions that support the laughter we hear.

The Infectious Nature of Laughter

Scientists found that when a funny phrase is paired with laughter there is a greater response in the brain than the identical word when accompanied by a non-emotional sound.

"This activation occurred in parts of the brain that you would use to contort your face into a smile or a laugh," she says.

It means we are not just reacting to humorous jokes, they are reacting to the laughter that accompanies them.

Laughter, says the professor, can be contagious.

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

"People laugh more when you know people," she notes, "and you laugh further when you are fond of them or care for them."

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

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

The Search for the Perfect Cracker Joke

Will we ever find the ultimate joke?

Probably not, but that has not prevented researchers from trying to.

In 2001, a psychologist established a research project for the world's funniest joke.

More than tens of thousands of jokes submitted, with ratings lodged by hundreds of thousands of participants globally, he has a better understanding than most as to what works and what fails.

The perfect festive cracker pun needs to be short, he says.

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

The increasingly "awful" the joke, he states the better.

"This is because if no-one laughs – it's the joke's shortcoming, not your own.

"The fascinating part about the Christmas cracker puns is that none of us find them humorous.

"That's a shared experience around the table and I believe it's wonderful."