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"What was the price did Santa'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 session with a company that makes supplies for social events. Its catalogue features Christmas crackers.

The company's founder grins, nearly apologetically at the gag. But the pun has been selected and will feature in future crackers.

"The success is gauged by the gag by the volume of groans and the intensity of the groans around the table," she says.

The key to a great Christmas cracker joke is not the identical as a good joke per se. It is all about the context - in this instance, the communal laughter of the holiday dinner table with elders, children and possibly friends.

"You want the joke to be something that unites the child together with the 80-year-old," she adds.

The Science Of Shared Laughter

Coming together to enjoy shared amusement is not only ancient, scientists argue, it is probably to be pre-human.

"So when you are laughing with others at the Christmas table you are dropping into what's very likely a really ancient mammal play sound," explains a neuroscience expert.

Communal amusement, she says, helps make and maintain social connections between individuals.

Scientists have discovered that a absence of such interactions can seriously damage mental and physical well-being.

"The people you converse with, and share laughter with, it results in enhanced levels of 'happy chemical' uptake," she adds.

These natural chemicals are the brain's "happy chemicals" and are produced both to alleviate tension and discomfort and in response to pleasurable experiences, such as laughing with loved ones over a particularly terrible Christmas cracker gag.

"You're not just chuckling at a silly joke with a holiday cracker," she says. "You are in fact doing a lot of the really vital task of making, maintaining the connections you have with those you care about."

What Happens In the Mind?

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

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

Employing functional magnetic resonance imaging (fMRI), a kind of brain scanner which shows which areas of the brain are working harder, scientists have been able to map the regions that receive more blood flow.

The research entails imaging the minds of volunteer subjects and then subjecting them to a collection of funny phrases, paired with either a neutral sound, or recorded laughter.

"In the scanner we got a really fascinating pattern of neural activity," says the professor.

A joke stimulates not just the parts of the brain in charge of auditory processing and understanding speech, but also neural regions associated with both preparation and starting motion and those involved in vision and memory.

Put these elements as a whole, and people hearing a joke have a sophisticated series of neural responses that underpin the amusement we experience.

The Contagious Nature of Laughter

Researchers found that when a humorous phrase is combined with laughter there is a greater response in the mind than the identical word when accompanied by a non-emotional sound.

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

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

Laughter, according to the professor, can be contagious.

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

"People laugh harder when you are familiar with others," she notes, "and laughter increases more when you are fond of them or love them."

When it comes to Christmas cracker jokes, she says, the positive effect is more likely to be caused not by the joke in 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 together."

The Search for the Perfect Festive Pun

Will we ever discover the perfect joke?

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

In 2001, a psychologist set up a research search for the planet's funniest gag.

More than tens of thousands of gags later, with ratings lodged by hundreds of thousands of people globally, he has a clearer idea than many as to what works and what fails.

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

"They must also be bad jokes, puns that cause us to moan," he continues.

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

"The reason is that if no-one laughs – it's the joke's shortcoming, not yours.

"What's interesting about the holiday cracker puns is that none of us find them humorous.

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