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

This quip is greeted with moans that echo through a warehouse in the capital.

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

The company's owner grins, nearly sheepishly at the gag. But the joke has made the cut and will appear in upcoming crackers.

"You measure the joke by the number of moans and the intensity of the groans at the table," she explains.

The key to a great Christmas cracker pun is not the identical as a stand-up gag per se. It is entirely about the setting - in this instance, the shared laughter of the holiday meal with grandparents, children and possibly friends.

"The goal is for the joke to be something that brings the child in harmony with the grandparent," she adds.

The Neuroscience Behind Communal Laughter

Coming together to experience shared laughter is not only nothing new, scientists argue, it is likely to be older than humanity.

"Therefore when you are chuckling with people at the holiday table you are engaging in what's almost certainly a really ancient mammalian play vocalisation," says a professor.

Communal laughter, she says, helps forge and strengthen social bonds between people.

Researchers have discovered that a absence of these interactions can seriously damage both psychological and bodily health.

"Those you converse with, and laugh with, it leads to increased levels of 'happy chemical' release," the professor adds.

Endorphins are the body's "happy chemicals" and are released both to alleviate stress and pain and in response to enjoyable experiences, such as chuckling with loved ones over a particularly awful festive cracker joke.

"You're not just chuckling at a foolish pun with a Christmas cracker," she says. "You are actually doing a lot of the truly vital task of building, preserving the connections you have with the people you care about."

Which Happens Inside the Brain?

But what is actually taking place inside the mind when we hear a joke?

An awful lot occurs in response to humour, it transpires.

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

Testing involves imaging the brains of healthy participants and then subjecting them to a collection of humorous words, paired with either a non-emotional sound, or recorded chuckles.

"During the study we observed a really interesting activation pattern of activation," notes the neuroscientist.

A joke stimulates not just the parts of the brain in charge of auditory processing and interpreting speech, but also brain regions involved in both planning and initiating movement and those involved in sight and recall.

Put all of this together, and individuals listening to a pun have a complex series of neural reactions that underpin the amusement we experience.

The Contagious Power of Chuckles

Scientists discovered that when a humorous phrase is paired with chuckles there is a stronger reaction in the mind than the same phrase when accompanied by a neutral sound.

"This was in areas of the mind that you would employ to move your face into a smile or a laugh," she explains.

It means people are not just responding to humorous jokes, they are responding to the laughter that follows them.

Laughter, says the expert, can be infectious.

So what does this imply for the chuckles found around a holiday table?

"You laugh more when you are familiar with people," she says, "and you laugh more when you are fond of them or love them."

When it comes to festive cracker puns, she explains, the positive factor is more probable to be triggered not by the gag itself, but from the reaction to it.

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

The Quest for the Ideal Festive Pun

Is it possible to find the ultimate gag?

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

Years ago, a psychologist set up a scientific search for the world's most humorous joke.

Over tens of thousands of gags submitted, with scores lodged by 350,000 participants globally, he has a better idea than many as to what works and what fails.

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

"They must also be poor jokes, jokes that make us moan," he adds.

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

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

"What's interesting about the holiday cracker puns is that not one person considers them funny.

"That's a common moment at the table and I think it's wonderful."