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

This joke is greeted with moans that resonate through a storage facility in the capital.

This describes a joke-testing meeting with a firm that makes products for gatherings. Its catalogue features festive crackers.

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

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

The secret to a good Christmas cracker joke is not the same as a stand-up gag per se. It is all about the setting - in this instance, the shared laughter of the holiday dinner table with grandparents, children and possibly neighbours.

"The goal is for the gag to be a thing that brings the child in harmony with the 80-year-old," she adds.

The Neuroscience Of Communal Laughter

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

"So when you are laughing with people around the Christmas table you are dropping into what's almost certainly a really primordial mammalian social vocalisation," explains a neuroscience expert.

Shared amusement, she explains, helps forge and strengthen social bonds between people.

Scientists have found that a lack of these social exchanges can seriously damage both psychological and bodily health.

"Those you talk to, and share laughter with, it leads to enhanced amounts of 'happy chemical' release," she continues.

These natural chemicals are the body's "happy chemicals" and are released both to reduce stress and pain and in reaction to enjoyable activities, such as laughing with loved ones over a particularly awful Christmas cracker joke.

"You're not just chuckling at a foolish pun with a holiday cracker," she states. "You are actually doing a lot of the truly important work of making, maintaining the social bonds you have with the people you love."

What Happens In the Brain?

But what is truly happening within the mind when we listen to a joke?

An awful lot occurs in response to humour, it turns out.

Using functional magnetic resonance imaging (fMRI), a kind of neural imager which shows which parts of the brain are more active, researchers have been able to chart the regions that receive more blood.

Testing entails imaging the brains of healthy subjects and then subjecting them to a database of humorous words, paired with either a neutral sound, or pre-recorded laughter.

"In the scanner we got a very fascinating activation pattern of activation," says the neuroscientist.

A joke activates not just the parts of the mind in charge of auditory processing and understanding language, but also neural regions involved in both planning and initiating movement and those involved in sight and recall.

Put these elements as a whole, and individuals hearing a pun have a sophisticated set of brain reactions that underpin the laughter we hear.

The Infectious Nature of Chuckles

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

"This was in parts of the mind that you would employ to contort your expression into a grin or a laugh," she says.

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

Laughter, according to the expert, can be infectious.

So what does this mean for the chuckles heard around a Christmas gathering?

"People laugh harder when you know people," she says, "and you laugh further when you like them or care for them."

When it comes to festive cracker jokes, she explains, the positive effect is more probable to be triggered not by the joke itself, but from the reaction to it.

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

The Search for the Ideal Festive Pun

Is it possible to discover the ultimate joke?

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

Years ago, a professor established a scientific project for the world's most humorous gag.

Over 40,000 gags submitted, with ratings provided by hundreds of thousands of participants globally, he has a better understanding than most as to what works and what fails.

The ideal festive cracker joke must be brief, he says.

"But they also be poor jokes, puns that make us groan," he continues.

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

"The reason is that if nobody finds it funny – it's the gag's fault, not your own.

"What's interesting about the Christmas cracker jokes is that not one person find them funny.

"That's a shared moment around the gathering and I believe it's wonderful."