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

This quip is greeted with moans that echo through a storage facility in London.

We're at a joke-testing session with a company that produces supplies for gatherings. Its catalogue features festive crackers.

The firm's founder smiles, almost apologetically at the gag. But the pun has made the cut and will feature in upcoming crackers.

"The success is gauged by the joke by the volume of moans and the intensity of the groans around the table," she explains.

The secret to a good Christmas cracker joke is not the same as a stand-up joke in itself. It is entirely about the setting - in this case, the communal laughter of the Christmas dinner table with elders, children and possibly friends.

"The goal is for the gag to be a thing that unites the eight-year-old together with the grandparent," she adds.

The Neuroscience Behind Shared Laughter

Gathering to experience communal amusement is not only ancient, scientists say, it is likely 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 social sound," says a professor.

Communal amusement, she says, helps make and maintain social bonds between individuals.

Scientists have found that a lack of these social exchanges can significantly damage mental and physical well-being.

"Those you talk to, and share laughter with, it leads to increased levels of 'happy chemical' release," she adds.

Endorphins are the brain's "happy chemicals" and are produced both to reduce tension and discomfort and in reaction to enjoyable activities, such as laughing with friends over a particularly awful festive cracker joke.

"You're not just chuckling at a silly pun with a Christmas cracker," she says. "You are in fact doing a lot of the truly vital task of building, preserving the social bonds you have with the people you love."

Which Happens In the Mind?

But what is actually happening inside the brain when we listen to a gag?

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

Employing functional magnetic resonance imaging (fMRI), a kind of brain scanner which shows which parts of the brain are working harder, researchers have been able to chart the regions that get more blood.

The research involves imaging the minds of volunteer subjects and then exposing them to a collection of humorous phrases, paired with either a neutral sound, or pre-recorded laughter.

"In the scanner we observed a very fascinating activation pattern of neural activity," says the neuroscientist.

A joke activates not just the parts of the mind responsible for auditory processing and understanding language, but also neural areas involved in both preparation and starting motion and those involved in sight and recall.

Combine all of this as a whole, and individuals hearing a pun have a sophisticated set of neural reactions that underpin the laughter we experience.

The Infectious Nature of Laughter

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

"This activation occurred in areas of the brain that you would use to contort your expression into a smile or a chuckle," the professor explains.

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

Laughter, according to the expert, can be contagious.

So what does this mean for the laughter found around a Christmas gathering?

"People laugh harder when you know others," she notes, "and laughter increases further when you are fond of them or care for them."

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

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

The Quest for the Ideal Festive Pun

Is it possible to discover the ultimate joke?

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

In 2001, a professor established a research project for the planet's funniest joke.

More than 40,000 gags submitted, with scores lodged by hundreds of thousands of people around the world, he has a better idea than most as to what succeeds and what does not.

The perfect Christmas cracker joke needs to be brief, he says.

"But they also need to be poor gags, jokes that make us moan," he continues.

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

"The reason is that if no-one finds it funny – it's the gag's shortcoming, not yours.

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

"That's a common moment at the table and I think it's wonderful."