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

This quip is greeted with groans that echo through a storage facility in London.

We're at a joke-testing session with a company that produces products for social events. Its catalogue features festive crackers.

The firm's founder smiles, nearly apologetically at the joke. But the joke has been selected and will feature in future crackers.

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

The secret to a great holiday cracker joke is not the same as a stand-up gag per se. It is entirely about the context - in this instance, the communal amusement of the Christmas meal with grandparents, children and possibly neighbours.

"You want the gag to be something that brings the child in harmony with the 80-year-old," she states.

The Neuroscience Behind Communal Laughter

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

"Therefore when you are laughing with others at the holiday table you are dropping into what's very likely a really primordial mammal play vocalisation," explains a neuroscience expert.

Communal laughter, she explains, helps forge and strengthen social connections between people.

Scientists have discovered that a lack of such interactions can seriously harm both psychological and bodily health.

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

These natural chemicals are the body's "happy chemicals" and are released both to alleviate tension and discomfort and in response to enjoyable activities, such as laughing with loved ones over a truly terrible Christmas cracker joke.

"It's not simply chuckling at a foolish joke with a holiday cracker," she says. "You are in fact performing a lot of the truly important work of making, maintaining the social bonds you have with those you care about."

Which Happens Inside the Brain?

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

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

Employing brain scanning technology, a kind of brain scanner which shows which areas of the mind are working harder, researchers have been able to map the regions that receive more blood.

Testing entails imaging the minds of healthy subjects and then subjecting them to a collection of funny phrases, paired with either a neutral sound, or recorded chuckles.

"In the scanner we got a very fascinating activation pattern of neural activity," notes the professor.

A joke activates not just the parts of the mind responsible for auditory processing and understanding language, but also neural regions involved in both preparation and starting motion and those linked to sight and memory.

Combine all of this together, and people listening to a pun have a sophisticated series of neural reactions that support the laughter we hear.

The Contagious Nature of Laughter

Researchers found that when a funny phrase is paired 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 mind that you would employ to move your expression into a smile or a chuckle," she says.

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

Laughter, says the expert, can be contagious.

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

"People laugh more when you are familiar with others," she says, "and laughter increases more when you like them or care for them."

When it comes to festive cracker puns, she explains, the positive factor 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 Ideal Cracker Joke

Is it possible to find the ultimate gag?

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

Years ago, a professor set up a scientific project for the world's most humorous joke.

Over tens of thousands of jokes submitted, with scores lodged by 350,000 people around the world, he has a clearer idea than most as to what works and what fails.

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

"They must also be poor jokes, jokes that cause us to moan," he adds.

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

"This is because if nobody finds it funny – it's the gag's shortcoming, not yours.

"The fascinating part about the Christmas cracker jokes is that none of us considers them funny.

"It creates a shared moment around the table and I think it's lovely."