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

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

We're at a humor-evaluation session with a firm that produces products for gatherings. Its repertoire includes festive crackers.

The firm's owner smiles, almost apologetically at the gag. But the pun has made the cut and will feature in future crackers.

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

The key to a good holiday cracker pun is not the same as a stand-up joke in itself. It is all about the context - in this instance, the shared amusement of the holiday meal with elders, children and possibly neighbours.

"The goal is for the gag to be something that brings the eight-year-old together with the 80-year-old," she adds.

The Science Behind Shared Laughter

Gathering to experience communal amusement is not only ancient, experts say, it is likely to be pre-human.

"Therefore when you are chuckling with others around the holiday dinner you are engaging in what's very likely a really primordial mammal social sound," says a professor.

Shared laughter, she explains, helps make and maintain social bonds between individuals.

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

"The people you talk to, and share laughter with, it leads to increased levels of 'happy chemical' release," she adds.

Endorphins are the brain's "happy chemicals" and are released both to reduce stress and pain and in response to pleasurable activities, such as laughing with loved ones over a particularly terrible festive cracker joke.

"You're not just laughing at a foolish joke with a holiday cracker," the expert states. "You are in fact doing a lot of the really important work of making, maintaining the social bonds you have with the people you love."

Which Happens In the Mind?

But what is truly happening within the brain when we listen to a gag?

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

Employing brain scanning technology, a type of brain scanner which indicates which parts of the brain are working harder, scientists have been able to chart the regions that get more blood.

Testing entails scanning the minds of volunteer participants and then subjecting them to a collection of funny phrases, paired with either a non-emotional sound, or recorded laughter.

"In the scanner we observed a really interesting activation pattern of activation," notes the neuroscientist.

A gag stimulates not just the parts of the mind responsible for auditory processing and understanding language, but also brain regions involved in both preparation and initiating movement and those linked to sight and memory.

Combine all of this as a whole, and individuals listening to a pun have a complex set of brain reactions that support the laughter we experience.

The Infectious Power of Chuckles

Researchers found that when a funny phrase is paired with chuckles there is a greater reaction in the mind than the identical word when followed by a non-emotional sound.

"This activation occurred in areas of the brain that you would use to move your face into a smile or a laugh," the professor says.

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

Amusement, according to the expert, can be infectious.

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

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

When it comes to Christmas cracker jokes, she says, the feel-good effect is more probable to be caused not by the gag itself, but from the response to it.

"The laughter is key. The gag is the terrible holiday cracker pun, and it's just a pretext to laugh as a group."

The Quest for the Ideal Festive Pun

Is it possible to find the ultimate joke?

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

Years ago, a professor set up a research project for the planet's most humorous gag.

Over 40,000 gags submitted, with ratings provided by hundreds of thousands of people around the world, he has a clearer understanding than many as to what succeeds and what fails.

The perfect Christmas cracker joke must be short, he says.

"They must also need to be bad gags, puns that make us groan," he adds.

The more "terrible" the joke, he says the more effective.

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

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

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