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

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

We're at a humor-evaluation meeting with a firm that produces supplies for gatherings. Its repertoire includes Christmas crackers.

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

"You measure the gag by the volume of groans and the loudness of the groans around the table," the founder explains.

The key to a good Christmas cracker pun is not the same as a good joke per se. It is all about the context - in this instance, the shared amusement of the Christmas meal with grandparents, kids and possibly friends.

"You want the joke to be a thing that unites the eight-year-old together with the 80-year-old," she adds.

The Neuroscience Behind Shared Laughter

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

"Therefore when you are laughing with people at the Christmas dinner you are dropping into what's very likely a truly ancient mammal play vocalisation," explains a neuroscience expert.

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

Researchers have found that a lack of these interactions can significantly harm mental and physical well-being.

"The people you converse with, and share laughter with, it results in increased amounts of 'happy chemical' release," the professor continues.

These natural chemicals are the brain's "happy chemicals" and are produced both to reduce tension and discomfort and in reaction to enjoyable activities, such as laughing with loved ones over a particularly awful Christmas cracker gag.

"You're not just chuckling at a foolish pun with a holiday cracker," she says. "You are in fact doing a lot of the truly vital work of building, preserving the connections you have with those you care about."

Which Happens In the Mind?

But what is truly happening within the brain when we hear a gag?

A tremendous amount happens in reaction to humour, it turns out.

Using functional magnetic resonance imaging (fMRI), a kind of neural imager which shows which parts of the brain are working harder, scientists have been able to map the regions that get more blood flow.

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

"In the scanner we observed a really fascinating pattern of neural activity," notes the professor.

A joke stimulates not just the parts of the brain in charge of auditory processing and interpreting language, but also neural regions involved in both planning and starting motion and those linked to vision and recall.

Combine all of this as a whole, and people listening to a joke have a complex series of neural responses that underpin the laughter we hear.

The Contagious Power of Laughter

Scientists found that when a funny phrase is paired with chuckles there is a greater reaction in the mind than the identical phrase when accompanied by a non-emotional sound.

"This was in areas of the mind that you would use to contort your expression into a smile or a laugh," she explains.

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

Laughter, says the expert, can be contagious.

So what does this imply for the laughter found at a Christmas table?

"You laugh harder when you are familiar with others," she says, "and you laugh more when you are fond of them or care for them."

When it comes to festive cracker puns, she explains, the positive effect is more likely to be triggered not by the gag in itself, but from the response to it.

"It's the laughter. The joke is the dreadful Christmas cracker joke, and it's just a pretext to chuckle together."

The Search for the Perfect Cracker Joke

Will we ever discover the ultimate joke?

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

In 2001, a psychologist set up a scientific search for the world's funniest gag.

More than tens of thousands of jokes submitted, with scores lodged by 350,000 people globally, he has a better idea than most as to what succeeds and what does not.

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

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

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

"This is because if no-one laughs – it's the gag's shortcoming, not yours.

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

"That's a common moment around the table and I believe it's wonderful."