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"What was the price did Santa's sled cost? Nothing, it was on the house."

This one-liner is met by groans that resonate through a storage facility in the capital.

We're at a joke-testing session with a firm that produces products for social events. Its catalogue features Christmas crackers.

The company's owner grins, almost sheepishly at the joke. But the pun has made the cut and will feature in future crackers.

"You measure the joke by the volume of moans and the intensity of the groans around the table," she says.

The secret to a good Christmas cracker joke is not the same as a good gag per se. It is entirely about the setting - in this case, the shared amusement of the holiday dinner table with grandparents, 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

Gathering to enjoy communal amusement is not only nothing new, experts argue, it is likely to be older than humanity.

"Therefore when you are chuckling with others at the holiday table you are dropping into what's very likely a truly primordial mammal social vocalisation," explains a neuroscience expert.

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

Researchers have found that a lack of such interactions can seriously damage both psychological and bodily health.

"The people you converse with, and laugh with, it leads to enhanced levels of endorphin release," the professor adds.

These natural chemicals are the brain's "feel-good compounds" and are produced both to reduce stress and pain and in reaction to pleasurable activities, such as chuckling with loved ones over a truly awful Christmas cracker joke.

"It's not simply chuckling at a foolish joke with a Christmas cracker," she says. "You are actually performing a lot of the really vital task of building, preserving the connections you have with those you love."

What Occurs In the Mind?

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

A tremendous amount happens in response to comedy, it turns out.

Using functional magnetic resonance imaging (fMRI), a kind of brain scanner which indicates which areas of the brain are more active, scientists have been able to map the areas that receive more blood flow.

The research involves imaging the minds of volunteer subjects and then subjecting them to a database of funny words, paired with either a neutral sound, or pre-recorded laughter.

"In the scanner we got a really fascinating activation pattern of activation," says the professor.

A joke stimulates not just the parts of the mind responsible for hearing and understanding language, but also neural regions associated with both planning and starting movement and those involved in vision and recall.

Combine all of this together, and individuals hearing a pun have a complex series of brain reactions that underpin the amusement we experience.

The Infectious Nature of Chuckles

Scientists discovered that when a funny word is paired with chuckles there is a stronger response in the mind than the identical phrase when accompanied 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," she explains.

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

Amusement, says the professor, can be infectious.

So what does this imply for the chuckles heard around a holiday gathering?

"You laugh more when you are familiar with people," she notes, "and you laugh more when you like them or love them."

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

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

The Quest for the Perfect Cracker Joke

Is it possible to discover the ultimate gag?

Probably not, but that has not prevented researchers from trying to.

Years ago, a professor set up a research project for the world's funniest gag.

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

The ideal festive cracker pun needs to be short, he explains.

"But they also need to be bad gags, puns that cause us to groan," he continues.

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

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

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

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