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"How much did Father Christmas'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 includes festive crackers.

The firm's owner grins, nearly apologetically at the gag. But the pun has been selected and will feature in future crackers.

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

The secret to a great holiday cracker joke is not the identical as a stand-up gag in itself. It is entirely about the setting - in this instance, the shared laughter of the Christmas dinner table with elders, children and potentially neighbours.

"You want the joke to be something that brings the eight-year-old together with the 80-year-old," she adds.

The Science Behind Shared Amusement

Gathering to experience shared laughter is not only nothing new, experts say, it is likely to be pre-human.

"Therefore when you are laughing with others around the Christmas dinner you are dropping into what's almost certainly a truly ancient mammal social vocalisation," says a neuroscience expert.

Communal laughter, she explains, helps forge and strengthen social connections between individuals.

Researchers have found that a absence of these interactions can seriously damage mental and physical well-being.

"Those you converse with, and share laughter with, it leads to increased levels of 'happy chemical' uptake," she continues.

Endorphins are the brain's "happy chemicals" and are released both to reduce tension and discomfort and in reaction to enjoyable experiences, such as chuckling with friends over a particularly terrible Christmas cracker gag.

"It's not simply chuckling at a foolish joke with a Christmas cracker," she says. "You are actually doing a lot of the truly vital work of making, maintaining the social bonds you have with those you care about."

Which Occurs In the Mind?

But what is actually taking place inside the brain when we hear a joke?

An awful lot occurs in response to comedy, it turns out.

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

Testing involves scanning the brains of volunteer subjects and then subjecting them to a collection of humorous words, paired with either a non-emotional sound, or recorded chuckles.

"In the scanner we observed a really interesting activation pattern of neural activity," says the professor.

A gag activates not just the areas of the brain in charge of hearing and understanding speech, but also brain areas involved in both preparation and starting motion and those involved in sight and memory.

Put all of this as a whole, and people hearing a joke have a complex series of brain reactions that underpin the laughter we experience.

The Contagious Nature of Laughter

Researchers found that when a humorous word is paired with chuckles there is a greater reaction in the mind than the identical word when followed by a non-emotional sound.

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

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

Amusement, says the professor, can be infectious.

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 you laugh more when you are fond of them or care for them."

When it comes to Christmas 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.

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

The Quest for the Perfect Festive Pun

Will we ever discover the perfect joke?

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

In 2001, a professor established a scientific project for the planet's most humorous gag.

Over tens of thousands of jokes submitted, with scores lodged by hundreds of thousands of people around the world, he has a clearer understanding than many as to what succeeds and what fails.

The ideal festive cracker joke needs to be short, he explains.

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

The more "terrible" the gag, he states the more effective.

"The reason is that if nobody laughs – it's the gag's shortcoming, not yours.

"The fascinating part about the Christmas cracker puns is that not one person find them humorous.

"That's a common moment around the gathering and I believe it's lovely."