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

This one-liner is greeted with moans that resonate through a warehouse in London.

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

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

"You measure the joke by the number of groans and the intensity of the groans around the table," she explains.

The secret to a great holiday cracker joke is not the same as a good gag per se. It is all about the setting - in this case, the communal laughter of the holiday meal with elders, children and potentially friends.

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

The Science Of Shared Laughter

Gathering to enjoy shared laughter is not only ancient, experts say, it is likely to be older than humanity.

"So when you are chuckling with others at the holiday table you are dropping into what's very likely a truly ancient mammalian social sound," explains a professor.

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

Scientists have found that a lack of such social exchanges can seriously damage mental and physical health.

"The people you talk to, and share laughter with, it results in enhanced levels of endorphin uptake," she adds.

These natural chemicals are the body's "happy chemicals" and are produced both to reduce tension and discomfort and in reaction to enjoyable experiences, such as laughing with loved ones over a truly terrible festive cracker joke.

"It's not simply chuckling at a foolish joke with a Christmas cracker," she says. "You are in fact performing a lot of the really important task of making, maintaining the social bonds you have with the people you care about."

Which Occurs In the Mind?

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

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

Employing functional magnetic resonance imaging (fMRI), a kind of neural imager which indicates which parts of the brain are working harder, researchers have been able to chart the areas that receive more blood.

Testing entails scanning the brains of healthy subjects and then subjecting them to a collection of funny phrases, accompanied by either a neutral sound, or recorded chuckles.

"During the study we got a really interesting activation pattern of neural activity," says the neuroscientist.

A joke activates not just the areas of the brain responsible for hearing and understanding speech, but also neural areas associated with both preparation and initiating movement and those linked to vision and recall.

Put these elements together, and people hearing a joke have a sophisticated set of brain reactions that support the laughter we experience.

The Contagious Nature of Laughter

Researchers discovered that when a humorous phrase is paired with laughter there is a stronger reaction in the mind than the identical phrase when accompanied by a non-emotional sound.

"This activation occurred in areas of the mind that you would employ to move your face into a grin or a chuckle," she explains.

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

Amusement, according to the expert, can be contagious.

So what does this imply for the laughter found at a Christmas table?

"People laugh more when you are familiar with others," she notes, "and laughter increases more when you like them or care for them."

When it comes to festive cracker jokes, she says, the positive effect is more probable to be triggered not by the joke itself, but from the reaction to it.

"The laughter is key. The joke is the dreadful Christmas cracker pun, and it's just a pretext to chuckle together."

The Quest for the Ideal Cracker Joke

Is it possible to discover the perfect joke?

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

Years ago, a professor set up a research search for the world's funniest joke.

Over 40,000 jokes submitted, with scores provided by hundreds of thousands of participants around the world, he has a clearer understanding 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 poor gags, puns that cause us to moan," he continues.

The increasingly "terrible" the gag, he states the better.

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

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

"It creates a common moment around the table and I think it's lovely."