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

This quip is met by groans that resonate through a warehouse in the capital.

This describes a humor-evaluation meeting with a firm that produces supplies for social events. Its repertoire features festive crackers.

The company's owner smiles, almost sheepishly at the joke. But the joke has been selected and will appear in future crackers.

"You measure the gag by the volume of groans and the loudness of the groans around the table," she says.

The key to a good Christmas cracker pun is not the same as a good gag in itself. It is entirely about the context - in this case, the communal laughter of the Christmas dinner table with grandparents, children and potentially neighbours.

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

The Neuroscience Behind Shared Amusement

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

"Therefore when you are chuckling with people around the Christmas table you are dropping into what's very likely a really primordial mammal social sound," says a neuroscience expert.

Communal laughter, she says, aids in make and maintain social connections between people.

Scientists have found that a lack of these social exchanges can significantly harm mental and physical well-being.

"Those you converse with, and laugh with, it results in enhanced levels of 'happy chemical' uptake," she continues.

Endorphins are the body's "happy chemicals" and are released both to reduce stress and pain and in reaction to pleasurable activities, such as laughing with friends over a truly awful Christmas cracker gag.

"It's not simply laughing at a silly joke with a holiday cracker," she says. "You are actually performing a lot of the truly vital task of making, maintaining the connections you have with those you love."

What Occurs Inside the Mind?

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

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

Using functional magnetic resonance imaging (fMRI), a type of neural imager which indicates which parts of the brain are more active, scientists have been able to chart the regions that get more blood flow.

The research entails scanning the minds of volunteer participants and then subjecting them to a collection of humorous words, paired with either a neutral sound, or recorded laughter.

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

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

Put these elements together, and individuals hearing a pun have a complex set of brain responses that underpin the amusement we hear.

The Infectious Power of Laughter

Scientists discovered that when a humorous word is combined with chuckles there is a stronger response in the brain than the identical phrase when followed by a non-emotional sound.

"This activation occurred in areas of the mind that you would employ to move your face into a grin or a laugh," she says.

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

Laughter, says the professor, can be infectious.

So what does this mean for the laughter heard around a holiday gathering?

"People laugh harder when you know people," she says, "and you laugh more when you are fond of them or love them."

When it comes to Christmas cracker puns, she says, the positive factor is more likely to be caused not by the gag in itself, but from the reaction to it.

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

The Search for the Perfect Cracker Joke

Is it possible to find the ultimate gag?

Likely not, but that has not prevented researchers from attempting to.

Years ago, a professor set up a scientific project for the planet's funniest gag.

Over 40,000 gags submitted, with scores provided by 350,000 participants globally, he has a better idea than most as to what works and what fails.

The perfect festive cracker joke needs to be brief, he says.

"They must also be poor gags, puns that cause us to groan," he continues.

The more "terrible" the gag, he states the more effective.

"This is because if nobody finds it funny – it's the joke's fault, not yours.

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

"It creates a common experience around the gathering and I think it's lovely."