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

This one-liner is met by groans that echo through a warehouse in the capital.

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

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

"The success is gauged by the joke by the number of groans and the loudness of the groans around the table," she says.

The secret to a great Christmas cracker pun is not the same as a stand-up gag in itself. It is all about the setting - in this instance, the communal amusement of the holiday dinner table with elders, children and possibly neighbours.

"You want the joke to be a thing that brings the child in harmony with the 80-year-old," she states.

The Neuroscience Behind Shared Laughter

Coming together to experience shared laughter is not only ancient, experts say, it is likely to be pre-human.

"Therefore when you are laughing with people at the Christmas table you are engaging in what's very likely a truly ancient mammal play sound," explains a neuroscience expert.

Shared amusement, she says, aids in forge and strengthen social connections between individuals.

Researchers have discovered that a lack of these social exchanges can significantly harm both psychological and bodily well-being.

"Those you talk to, and laugh with, it results in increased amounts of endorphin uptake," the professor adds.

Endorphins are the brain's "feel-good compounds" and are produced both to reduce stress and pain and in reaction to pleasurable experiences, such as chuckling with loved ones over a particularly terrible festive cracker gag.

"You're not just chuckling at a foolish pun with a holiday cracker," she says. "You are actually performing a lot of the truly vital work of building, preserving the social bonds you have with those you care about."

Which Happens In the Mind?

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

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

Employing brain scanning technology, a kind of brain scanner which shows which parts of the brain are more active, scientists have been able to chart the areas that receive more blood flow.

The research entails imaging the brains of volunteer participants and then subjecting them to a collection of humorous phrases, paired with either a neutral sound, or recorded chuckles.

"In the scanner we got a really interesting activation pattern of neural activity," notes the neuroscientist.

A joke stimulates not just the areas of the mind responsible for hearing and interpreting speech, but also brain areas associated with both planning and starting motion and those linked to sight and recall.

Put these elements together, and people listening to a pun have a sophisticated set of brain reactions that underpin the amusement we hear.

The Infectious Nature of Chuckles

Researchers found that when a humorous word is paired with laughter there is a greater reaction in the brain than the identical phrase when accompanied by a neutral sound.

"This activation occurred in parts of the mind that you would employ to move your expression into a grin or a chuckle," the professor says.

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

Laughter, says the professor, can be infectious.

So what does this imply for the laughter found at a Christmas table?

"You laugh harder when you are familiar with people," she says, "and laughter increases further when you are fond of them or care for them."

When it comes to festive cracker puns, she explains, the feel-good factor is more probable to be caused not by the joke itself, but from the reaction to it.

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

The Search for the Ideal Festive Pun

Is it possible to find the perfect joke?

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

In 2001, a psychologist set up a research search for the planet's funniest joke.

Over 40,000 jokes submitted, with scores provided by 350,000 people globally, he has a clearer understanding than many as to what works and what does not.

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

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

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

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

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

"It creates a shared experience around the table and I believe it's lovely."