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"What was the price did Father Christmas's sleigh cost? Zero, it was on the house."

This quip is greeted with groans that resonate through a storage facility in the capital.

We're at a joke-testing session with a firm that produces supplies for social events. Its repertoire includes Christmas crackers.

The company's founder grins, nearly sheepishly at the joke. But the pun has made the cut and will appear in upcoming crackers.

"The success is gauged by the joke by the volume of moans and the loudness of the groans at the table," the founder says.

The key to a good Christmas cracker joke is not the identical as a stand-up joke in itself. It is entirely about the context - in this instance, the shared amusement of the Christmas meal with elders, children and potentially friends.

"You want the gag to be a thing that brings the eight-year-old together with the grandparent," she adds.

The Neuroscience Behind Communal Amusement

Gathering to enjoy shared laughter is not only nothing new, scientists say, it is likely to be pre-human.

"So when you are laughing with people at the Christmas dinner you are dropping into what's almost certainly a really ancient mammalian social sound," says a professor.

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

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

"The people you converse with, and laugh with, it results in increased amounts of 'happy chemical' uptake," she continues.

Endorphins are the brain's "feel-good compounds" and are released both to alleviate tension and discomfort and in response to pleasurable experiences, such as laughing with loved ones over a particularly terrible festive cracker gag.

"It's not simply laughing at a silly pun with a holiday cracker," she says. "You are in fact performing a lot of the truly vital task of building, preserving the connections you have with the people you care about."

Which Happens In the Mind?

But what is truly taking place inside the brain when we hear a joke?

An awful lot happens in reaction to humour, it transpires.

Employing brain scanning technology, a type of neural imager which shows which areas of the brain are more active, researchers have been able to chart the areas that get more blood.

Testing involves imaging the brains of healthy participants and then subjecting them to a database of humorous phrases, accompanied by either a neutral sound, or recorded chuckles.

"During the study we observed a really interesting pattern of activation," says the professor.

A gag stimulates not just the areas of the mind in charge of auditory processing and understanding speech, but also neural regions associated with both planning and initiating movement and those linked to vision and memory.

Put all of this together, and individuals listening to a joke have a complex set of neural reactions that support the amusement we experience.

The Infectious Nature of Laughter

Scientists discovered that when a humorous word is paired with chuckles there is a greater response in the mind than the identical phrase when followed by a non-emotional sound.

"This was in parts of the brain that you would employ to contort your face into a smile or a chuckle," the professor says.

It means we are not just reacting to funny words, they are responding to the amusement that accompanies them.

Amusement, says the professor, can be infectious.

So what does this mean for the laughter heard around a Christmas table?

"People laugh more when you know others," she says, "and you laugh more when you are fond of them or care for them."

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

"The laughter is key. The gag is the dreadful holiday cracker pun, and it's just a reason to chuckle as a group."

The Search for the Perfect Cracker Joke

Will we ever discover the ultimate joke?

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

In 2001, a professor set up a scientific project for the world's funniest gag.

Over tens of thousands of jokes later, with ratings provided by 350,000 participants globally, he has a clearer understanding than many as to what works and what fails.

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

"They must also be bad jokes, puns that cause us to moan," he adds.

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

"The reason is that if no-one laughs – it's the joke's shortcoming, not yours.

"The fascinating part about the Christmas cracker puns is that none of us find them humorous.

"That's a shared moment around the table and I think it's lovely."