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"How much did Father Christmas's sled cost? Zero, it was on the house."

This quip is met by moans that resonate through a storage facility in the capital.

We're at a joke-testing meeting with a company that makes supplies for social events. Its repertoire includes Christmas crackers.

The firm's owner grins, nearly sheepishly at the gag. But the joke has been selected and will feature in upcoming crackers.

"You measure the gag by the volume of groans and the intensity of the groans around the table," she explains.

The secret to a good holiday cracker joke is not the same as a good joke in itself. It is entirely about the context - in this instance, the communal laughter of the Christmas dinner table with grandparents, kids and potentially friends.

"The goal is for the gag to be a thing that unites the eight-year-old in harmony with the 80-year-old," she states.

The Neuroscience Behind Shared Amusement

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

"Therefore when you are chuckling with people at the holiday dinner you are engaging in what's very likely a truly primordial mammal play sound," explains a professor.

Shared laughter, she says, helps forge and strengthen social connections between people.

Researchers have found that a lack of these social exchanges can seriously harm both psychological and bodily well-being.

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

These natural chemicals are the brain's "happy chemicals" and are released both to alleviate tension and discomfort and in reaction to pleasurable experiences, such as chuckling with loved ones over a truly awful Christmas cracker gag.

"It's not simply chuckling at a silly joke with a Christmas cracker," she states. "You are in fact performing a lot of the really vital task of making, maintaining the social bonds you have with those you love."

What Happens Inside the Brain?

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

A tremendous amount occurs in reaction to comedy, it transpires.

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

The research involves scanning the brains of healthy participants and then subjecting them to a collection of funny phrases, paired with either a neutral sound, or recorded chuckles.

"During the study we got a really fascinating pattern of neural activity," notes the neuroscientist.

A gag activates not just the areas of the brain responsible for auditory processing and interpreting speech, but also brain areas involved in both planning and starting motion and those involved in vision and memory.

Combine all of this together, and people hearing a joke have a sophisticated series of brain reactions that underpin the laughter we hear.

The Infectious Power of Chuckles

Scientists found that when a humorous word is combined with laughter there is a stronger response in the brain than the same phrase when accompanied by a neutral sound.

"This activation occurred in parts of the brain that you would employ to move your expression into a smile or a laugh," the professor explains.

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

Laughter, says the professor, can be infectious.

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

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

When it comes to Christmas cracker jokes, she explains, the feel-good factor is more probable to be caused not by the joke itself, but from the response to it.

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

The Quest for the Perfect Festive Pun

Is it possible to find the ultimate gag?

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

In 2001, a psychologist established a research search for the planet's most humorous joke.

More than 40,000 jokes submitted, with scores provided by 350,000 people around the world, he has a clearer understanding than many as to what works and what fails.

The ideal Christmas cracker pun must be brief, he says.

"They must also be poor gags, puns that make us moan," he continues.

The increasingly "terrible" the gag, he says the better.

"The reason is that if no-one laughs – it's the gag's shortcoming, not your own.

"What's interesting about the Christmas cracker jokes is that none of us considers them humorous.

"That's a shared experience at the table and I think it's wonderful."