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

This joke is met by groans that resonate through a storage facility in the capital.

We're at a joke-testing meeting with a company that makes products for gatherings. Its repertoire includes festive crackers.

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

"The success is gauged by the gag by the volume of moans and the intensity of the groans at the table," she explains.

The secret to a good Christmas cracker pun is not the same as a good gag per se. It is entirely about the setting - in this instance, the shared laughter of the Christmas meal with grandparents, children and possibly neighbours.

"The goal is for the gag to be a thing that brings the child together with the grandparent," she adds.

The Neuroscience Behind Shared Laughter

Gathering to experience communal laughter is not only ancient, experts argue, it is likely to be pre-human.

"So when you are laughing with others around the holiday table you are dropping into what's almost certainly a truly ancient mammal social sound," says a professor.

Communal laughter, she says, helps forge and strengthen social connections between individuals.

Researchers have discovered that a lack of such social exchanges can seriously harm both psychological and bodily health.

"The people you talk to, and share laughter with, it results in increased amounts of 'happy chemical' uptake," she adds.

These natural chemicals are the brain's "feel-good compounds" and are produced both to reduce stress and pain and in response to enjoyable experiences, such as chuckling with friends over a particularly terrible Christmas cracker gag.

"You're not just chuckling at a silly joke with a Christmas cracker," the expert says. "You are actually performing a lot of the really vital work of building, preserving the social bonds you have with those you love."

Which Occurs In the Mind?

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

An awful lot happens in reaction to humour, it turns out.

Using functional magnetic resonance imaging (fMRI), a type of brain scanner which indicates which parts of the brain are working harder, researchers have been able to map the areas that get more blood flow.

Testing entails scanning the brains of volunteer subjects and then exposing them to a database of funny phrases, paired with either a neutral sound, or recorded laughter.

"In the scanner we observed a really interesting pattern of activation," notes the neuroscientist.

A gag stimulates not just the areas of the brain responsible for auditory processing and understanding language, but also brain regions associated with both planning and starting movement and those linked to sight and memory.

Combine these elements together, and people hearing a joke have a complex set of brain responses that underpin the amusement we hear.

The Contagious Power of Laughter

Scientists found that when a humorous word is combined with chuckles there is a greater response in the mind than the identical word when followed by a non-emotional sound.

"This was in areas of the brain that you would use to move your expression into a smile or a laugh," the professor explains.

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

Laughter, according to the expert, can be infectious.

So what does this mean for the chuckles found around a Christmas table?

"You laugh harder when you know others," she says, "and laughter increases further when you like them or care for them."

When it comes to Christmas cracker jokes, she explains, the positive effect is more probable to be triggered not by the gag itself, but from the response to it.

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

The Quest for the Perfect Cracker Joke

Is it possible to find the ultimate gag?

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

Years ago, a psychologist set up a scientific search for the planet's funniest joke.

Over 40,000 jokes later, with scores provided by hundreds of thousands of participants globally, he has a clearer understanding than many as to what succeeds and what does not.

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

"They must also need to be poor gags, puns that make us groan," he adds.

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

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

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

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