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"How much did Father Christmas's sleigh cost? Nothing, it was on the house."

This quip is met by groans that resonate through a warehouse in London.

We're at a joke-testing meeting with a company that makes products for gatherings. Its catalogue includes festive crackers.

The company's owner smiles, nearly sheepishly at the joke. But the joke has been selected and will appear in future crackers.

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

The key to a great holiday cracker pun is not the identical as a stand-up gag per se. It is all about the context - in this instance, the shared amusement of the holiday meal with grandparents, children and potentially friends.

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

The Neuroscience Of Communal Amusement

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

"So when you are laughing with people at the holiday dinner you are dropping into what's very likely a truly primordial mammalian social vocalisation," says a neuroscience expert.

Communal laughter, she says, helps forge and strengthen social connections between people.

Researchers have discovered that a lack of these interactions can significantly damage both psychological and bodily well-being.

"The people you talk to, and laugh with, it leads to increased amounts of 'happy chemical' release," the professor adds.

Endorphins are the body's "happy chemicals" and are released both to reduce stress and pain and in reaction 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 Christmas cracker," she states. "You are in fact performing a lot of the truly vital work of building, preserving the connections you have with the people you love."

What Happens In the Brain?

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

A tremendous amount happens in reaction to humour, it turns out.

Employing brain scanning technology, a type of brain scanner which indicates which parts of the brain are more active, researchers have been able to map the regions that get more blood.

Testing involves imaging the minds of healthy subjects and then subjecting them to a database of funny words, paired with either a non-emotional sound, or pre-recorded chuckles.

"During the study we got a very fascinating pattern of activation," notes the professor.

A joke stimulates not just the parts of the mind in charge of hearing and understanding language, but also neural areas involved in both planning and starting movement and those linked to sight and memory.

Put all of this as a whole, and individuals listening to a joke have a sophisticated series of neural reactions that support the laughter we hear.

The Infectious Nature of Chuckles

Scientists discovered that when a humorous word is combined with chuckles there is a stronger reaction in the mind than the identical word when accompanied by a non-emotional sound.

"This activation occurred in parts of the brain that you would use to move your face into a smile or a chuckle," she says.

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

Laughter, according to the professor, can be contagious.

So what does this imply for the laughter heard at a holiday table?

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

When it comes to festive cracker jokes, she explains, the feel-good effect is more likely to be triggered not by the joke itself, but from the reaction to it.

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

The Quest for the Perfect Cracker Joke

Will we ever find the ultimate joke?

Probably not, but that has not stopped experts from trying to.

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

Over tens of thousands of jokes later, with ratings lodged by 350,000 participants around the world, he has a clearer idea than many as to what succeeds and what does not.

The perfect Christmas cracker pun needs to be brief, he explains.

"But they also need to be poor gags, jokes that cause us to groan," he adds.

The increasingly "terrible" the gag, he says 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 puns is that none of us find them humorous.

"It creates a common experience at the table and I think it's wonderful."