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

This joke is met by groans that echo through a storage facility in the capital.

This describes a joke-testing session with a company that produces products for gatherings. Its repertoire features Christmas crackers.

The firm's founder grins, nearly sheepishly at the gag. But the pun has made the cut and will appear in future crackers.

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

The secret to a great holiday cracker joke is not the same as a good joke per se. It is entirely about the context - in this case, the shared laughter of the Christmas meal with grandparents, children and possibly friends.

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

The Neuroscience Behind Shared Laughter

Gathering to experience communal amusement is not only ancient, experts say, it is likely to be pre-human.

"Therefore when you are chuckling with others at the Christmas table you are engaging in what's almost certainly a truly ancient mammalian play vocalisation," says a professor.

Shared laughter, she says, helps forge and strengthen social bonds between individuals.

Scientists have discovered that a absence of such interactions can significantly damage both psychological and bodily well-being.

"Those you converse with, and share laughter with, it results in increased amounts of endorphin uptake," the professor continues.

Endorphins are the brain's "happy chemicals" and are produced both to alleviate tension and discomfort and in reaction to pleasurable experiences, such as chuckling with friends over a particularly awful Christmas cracker gag.

"You're not just chuckling at a silly pun with a holiday cracker," she states. "You are actually doing a lot of the really vital work of making, maintaining the social bonds you have with those you care about."

What Happens In the Mind?

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

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

Using functional magnetic resonance imaging (fMRI), a type of brain scanner which shows which areas of the brain are working harder, scientists have been able to map the regions that receive more blood.

Testing entails imaging the brains of healthy subjects and then subjecting them to a database of humorous words, accompanied by either a neutral sound, or pre-recorded laughter.

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

A gag stimulates not just the parts of the brain responsible for hearing and understanding language, but also neural areas associated with both preparation and starting motion and those linked to sight and recall.

Put all of this together, and people hearing a joke have a complex set of neural responses that underpin the laughter we hear.

The Contagious Nature of Laughter

Researchers found that when a humorous word is paired with laughter there is a stronger response in the brain than the same phrase when accompanied by a neutral sound.

"This was in parts of the brain that you would use to move your face into a grin or a chuckle," the professor explains.

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

Laughter, according to the professor, can be contagious.

So what does this imply for the chuckles heard around a Christmas table?

"You laugh more when you are familiar with others," she notes, "and laughter increases more when you are fond of them or love them."

When it comes to festive cracker jokes, she says, the feel-good factor is more probable to be triggered not by the joke in itself, but from the response to it.

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

The Search for the Perfect Cracker Joke

Is it possible to discover the perfect gag?

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

In 2001, a psychologist set up a scientific project for the world's most humorous joke.

Over tens of thousands of jokes later, with scores lodged by 350,000 participants globally, he has a better understanding than most as to what succeeds and what fails.

The perfect festive cracker pun must be short, he says.

"They must also be poor gags, puns that make us moan," he adds.

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

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

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

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