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

This one-liner is met by groans that echo through a storage facility in London.

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

The firm's owner grins, almost apologetically at the joke. But the pun has made the cut and will appear in future crackers.

"The success is gauged by the gag by the number of groans and the intensity of the groans at the table," the founder says.

The secret to a great holiday cracker pun is not the same as a good joke per se. It is entirely about the setting - in this instance, the communal amusement of the holiday meal with grandparents, kids and potentially neighbours.

"The goal is for the joke to be something that brings the child in harmony with the 80-year-old," she adds.

The Science Behind Shared Laughter

Gathering to enjoy communal laughter is not only ancient, experts say, it is probably to be pre-human.

"Therefore when you are chuckling with others around the holiday dinner you are dropping into what's almost certainly a really ancient mammalian play vocalisation," explains a neuroscience expert.

Communal amusement, she says, aids in forge and strengthen social bonds between people.

Researchers have found that a absence of such social exchanges can seriously damage both psychological and bodily well-being.

"Those you converse with, and laugh with, it leads to enhanced amounts of endorphin uptake," she adds.

These natural chemicals are the body's "happy chemicals" and are released both to reduce stress and pain and in response to pleasurable experiences, such as laughing with loved ones over a truly terrible Christmas cracker joke.

"It's not simply laughing at a silly joke with a holiday cracker," the expert states. "You are in fact doing a lot of the truly important work of making, maintaining the social bonds you have with those you love."

Which Occurs In the Brain?

But what is truly happening within the mind when we hear a joke?

A tremendous amount happens in reaction to comedy, it transpires.

Employing functional magnetic resonance imaging (fMRI), a type of neural imager which shows which parts of the brain are more active, researchers have been able to chart the areas that get more blood flow.

Testing involves scanning the brains of healthy subjects and then subjecting them to a collection of funny phrases, accompanied by either a non-emotional sound, or recorded laughter.

"During the study we observed a very fascinating activation pattern of activation," says the professor.

A gag activates not just the parts of the brain responsible for hearing and interpreting speech, but also brain areas involved in both preparation and starting motion and those linked to sight and memory.

Combine these elements as a whole, and people listening to a pun have a sophisticated series of neural reactions that underpin the laughter we experience.

The Contagious Power of Laughter

Researchers discovered that when a funny phrase is combined with laughter there is a stronger reaction in the brain than the same word when accompanied by a neutral sound.

"This was in areas of the mind that you would employ to contort your face into a grin or a chuckle," the professor explains.

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

Amusement, according to the professor, can be infectious.

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

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

When it comes to Christmas cracker jokes, she explains, the positive effect is more likely to be triggered not by the gag in itself, but from the response to it.

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

The Search for the Perfect Festive Pun

Is it possible to find the ultimate joke?

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

Years ago, a psychologist established a scientific project for the planet's most humorous joke.

Over tens of thousands of jokes later, with scores lodged by hundreds of thousands of people globally, he has a better idea than most as to what works and what fails.

The perfect festive cracker pun needs to be short, he says.

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

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

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

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

"It creates a common experience around the gathering and I believe it's lovely."