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

This one-liner is met by groans that resonate through a warehouse in London.

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

The company's owner grins, almost apologetically at the joke. But the pun has been selected and will feature in future crackers.

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

The secret to a great Christmas cracker joke is not the same as a stand-up joke per se. It is all about the context - in this case, the communal laughter of the Christmas meal with grandparents, kids and possibly neighbours.

"The goal is for the joke to be something that unites the child together with the grandparent," she states.

The Neuroscience Of Communal Laughter

Gathering to experience shared amusement is not only nothing new, scientists argue, it is probably to be pre-human.

"So when you are chuckling with others around the Christmas table you are dropping into what's very likely a truly primordial mammal play vocalisation," says a neuroscience expert.

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

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

"Those you converse with, and share laughter with, it results in increased levels of 'happy chemical' uptake," the professor adds.

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

"You're not just laughing at a silly joke with a holiday cracker," she states. "You are in fact performing a lot of the really important work of building, preserving the social bonds you have with the people you care about."

Which Occurs In the Mind?

But what is truly happening inside the mind when we hear a gag?

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

Employing functional magnetic resonance imaging (fMRI), a kind of neural imager which indicates which parts of the brain are more active, researchers have been able to map the regions that receive more blood flow.

The research entails scanning the minds of healthy participants and then exposing them to a collection of humorous phrases, paired with either a non-emotional sound, or recorded laughter.

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

A joke stimulates not just the areas of the brain in charge of hearing and understanding language, but also neural regions involved in both preparation and initiating motion and those linked to sight and recall.

Put all of this as a whole, and individuals hearing a joke have a complex set of brain responses that support the laughter we experience.

The Contagious Nature of Chuckles

Scientists discovered that when a humorous phrase is combined with laughter there is a greater reaction in the mind than the identical phrase when accompanied by a neutral sound.

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

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

Amusement, according to the expert, can be contagious.

So what does this imply for the chuckles found around a holiday table?

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

When it comes to Christmas cracker puns, she explains, the feel-good factor 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 dreadful Christmas cracker joke, and it's just a reason to chuckle together."

The Quest for the Perfect Cracker Joke

Is it possible to discover the ultimate gag?

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

Years ago, a professor established a scientific search for the world's most humorous joke.

Over tens of thousands of gags submitted, with scores lodged by hundreds of thousands of participants globally, he has a clearer idea than many as to what works and what fails.

The perfect festive cracker pun must be brief, he explains.

"They must also need to be bad jokes, jokes that make us moan," he adds.

The increasingly "terrible" the gag, he says the more effective.

"The reason is that if nobody finds it funny – it's the joke's shortcoming, not yours.

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

"It creates a shared experience at the gathering and I think it's lovely."