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

This quip is met by moans that echo through a storage facility in the capital.

This describes a joke-testing session with a firm that makes supplies for social events. Its repertoire includes Christmas crackers.

The firm's owner grins, almost sheepishly at the gag. But the joke has made the cut and will feature in future crackers.

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

The secret to a great holiday cracker pun is not the same as a stand-up gag per se. It is all about the setting - in this case, the shared amusement of the Christmas dinner table with grandparents, children and possibly neighbours.

"You want the joke to be something that brings the child in harmony with the 80-year-old," she states.

The Science Behind Shared Amusement

Gathering to enjoy shared laughter is not only nothing new, scientists say, it is probably to be older than humanity.

"So when you are laughing with others around the Christmas dinner you are engaging in what's almost certainly a really primordial mammal social vocalisation," says a neuroscience expert.

Shared laughter, she explains, helps make and maintain social connections between individuals.

Researchers have discovered that a lack of such interactions can seriously damage both psychological and bodily well-being.

"The people you talk to, and laugh with, it leads to enhanced amounts of endorphin release," the professor continues.

These natural chemicals are the body's "feel-good compounds" and are produced both to reduce tension and discomfort and in response to pleasurable activities, such as laughing with friends over a truly terrible festive cracker joke.

"It's not simply laughing at a foolish joke with a Christmas cracker," she says. "You are in fact doing a lot of the truly vital task of making, maintaining the social bonds you have with the people you love."

What Happens In the Mind?

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

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

Using functional magnetic resonance imaging (fMRI), a type of neural imager which shows which areas of the brain are more active, researchers have been able to chart the areas that receive more blood.

The research involves scanning the minds of volunteer subjects and then subjecting them to a collection of humorous words, paired with either a non-emotional sound, or recorded laughter.

"In the scanner we got a very interesting pattern of neural activity," notes the neuroscientist.

A gag stimulates not just the parts of the brain in charge of hearing and understanding speech, but also neural regions involved in both preparation and starting movement and those linked to sight and memory.

Combine these elements together, and individuals listening to a joke have a complex series of brain reactions that underpin the laughter we hear.

The Contagious Nature of Chuckles

Researchers discovered that when a humorous word is paired with chuckles there is a stronger response in the brain than the identical phrase when followed by a neutral sound.

"This activation occurred in areas of the mind that you would use to move your expression into a grin or a laugh," the professor says.

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

Amusement, says the professor, can be contagious.

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

"People laugh more when you know people," she says, "and you laugh further when you like them or love them."

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

"The laughter is key. The joke is the terrible holiday cracker pun, and it's just a reason to laugh together."

The Quest for the Perfect Cracker Joke

Will we ever find the ultimate joke?

Probably not, but that has not prevented experts from attempting to.

Years ago, a psychologist set up a scientific project for the planet's funniest gag.

More than tens of thousands of gags later, with scores lodged by 350,000 people globally, he has a better understanding than many as to what succeeds and what fails.

The ideal Christmas cracker pun must be brief, he explains.

"They must also be poor jokes, puns that make us groan," he continues.

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

"This is because if no-one finds it funny – it's the gag's fault, not yours.

"What's interesting about the holiday cracker puns is that not one person considers them funny.

"That's a shared moment at the gathering and I think it's lovely."