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

This one-liner is greeted with groans that echo through a warehouse in the capital.

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

The firm's owner grins, almost 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 number of groans and the loudness of the groans at the table," the founder explains.

The key to a good Christmas cracker joke is not the same as a stand-up gag per se. It is entirely about the setting - in this instance, the shared amusement of the Christmas dinner table with grandparents, kids and possibly neighbours.

"You want the gag to be a thing that brings the child in harmony with the grandparent," she adds.

The Neuroscience Behind Shared Amusement

Gathering to enjoy shared laughter is not only ancient, scientists argue, it is likely to be pre-human.

"Therefore when you are laughing with people around the Christmas table you are engaging in what's very likely a truly primordial mammalian social sound," explains a neuroscience expert.

Communal laughter, she explains, aids in forge and strengthen social connections between individuals.

Scientists have found that a absence of such social exchanges can significantly harm mental and physical health.

"Those you converse with, and share laughter with, it leads to increased levels of 'happy chemical' uptake," she continues.

These natural chemicals are the brain's "feel-good compounds" and are produced both to alleviate stress and pain and in reaction to pleasurable activities, such as laughing with friends over a particularly terrible festive cracker joke.

"You're not just chuckling at a silly joke with a Christmas cracker," the expert says. "You are actually doing a lot of the really vital task of making, maintaining the connections you have with the people you love."

What Happens Inside the Brain?

But what is actually taking place within the brain when we hear a joke?

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

Using brain scanning technology, a kind of neural imager which indicates which areas of the brain are working harder, researchers have been able to map the regions that receive more blood flow.

Testing entails imaging the minds of volunteer subjects and then subjecting them to a database of humorous words, paired with either a non-emotional sound, or recorded laughter.

"In the scanner we observed a really fascinating pattern of neural activity," says the neuroscientist.

A joke stimulates not just the parts of the mind responsible for auditory processing and interpreting speech, but also brain regions involved in both planning and starting motion and those involved in vision and memory.

Put all of this together, and individuals hearing a joke have a sophisticated series of brain responses that underpin the amusement we experience.

The Infectious Nature of Laughter

Researchers discovered that when a humorous phrase is paired with chuckles there is a greater reaction in the brain than the identical phrase when followed by a neutral sound.

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

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

Laughter, according to the expert, can be infectious.

So what does this imply for the chuckles heard at a Christmas gathering?

"You laugh harder when you are familiar with others," she notes, "and you laugh further when you like them or love them."

When it comes to festive cracker puns, she explains, the feel-good factor is more likely to be triggered not by the gag in itself, but from the response to it.

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

The Search for the Perfect Cracker Joke

Is it possible to find the perfect joke?

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

Years ago, a professor set up a research search for the planet's funniest joke.

More than tens of thousands of gags submitted, with scores provided by 350,000 participants globally, he has a clearer understanding than many as to what works and what fails.

The ideal Christmas cracker joke needs to be short, he explains.

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

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

"This is because if no-one finds it funny – it's the joke's shortcoming, not your own.

"The fascinating part about the holiday cracker jokes is that not one person considers them funny.

"That's a common experience at the table and I think it's lovely."