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"How much 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 meeting with a firm that produces products for social events. Its repertoire features Christmas crackers.

The company's owner smiles, almost sheepishly at the joke. But the pun has made the cut and will appear in future crackers.

"The success is gauged by the joke by the volume of groans and the loudness of the groans at the table," she explains.

The key to a good Christmas cracker joke is not the identical as a good joke in itself. It is entirely about the setting - in this case, the communal laughter of the holiday meal with grandparents, 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 Science Behind Shared Laughter

Coming together to experience communal laughter is not only nothing new, experts argue, it is probably to be pre-human.

"So when you are laughing with others at the Christmas table you are dropping into what's very likely a truly ancient mammal social vocalisation," says a neuroscience expert.

Shared amusement, she says, helps make and maintain social connections between individuals.

Researchers have found that a absence of such social exchanges can seriously damage mental and physical well-being.

"Those you converse with, and laugh with, it results in increased amounts of 'happy chemical' uptake," she continues.

These natural chemicals are the brain's "feel-good compounds" and are produced both to reduce stress and pain and in response to pleasurable activities, such as laughing with loved ones over a particularly awful Christmas cracker gag.

"It's not simply laughing at a foolish pun with a Christmas cracker," she says. "You are actually doing a lot of the truly important task of building, preserving the social bonds you have with the people you love."

Which Happens In the Brain?

But what is truly happening within the brain when we hear a joke?

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

Using functional magnetic resonance imaging (fMRI), a type of brain scanner which indicates which areas 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 participants and then subjecting them to a database of funny words, accompanied by either a non-emotional sound, or recorded laughter.

"During the study we got a very interesting activation pattern of activation," notes the neuroscientist.

A gag stimulates not just the parts of the brain in charge of auditory processing and interpreting speech, but also brain areas involved in both planning and starting motion and those linked to vision and memory.

Combine all of this together, and people listening to a joke have a sophisticated series of brain responses that support the amusement we experience.

The Contagious Nature of Chuckles

Scientists discovered that when a funny word is paired with chuckles there is a greater reaction in the brain than the identical word when accompanied by a non-emotional sound.

"This activation occurred in parts of the brain that you would employ to move your expression into a smile or a chuckle," she says.

It means people are not just responding to funny jokes, they are responding to the amusement that accompanies them.

Amusement, according to the professor, can be contagious.

So what does this mean for the laughter found at a holiday table?

"You laugh harder when you know others," she says, "and you laugh more when you like them or love them."

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

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

The Search for the Ideal Festive Pun

Is it possible to discover the perfect gag?

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

Years ago, a psychologist set up a research search for the planet's most humorous joke.

Over tens of thousands of jokes submitted, with scores lodged by 350,000 people globally, he has a better idea than many as to what works and what fails.

The perfect Christmas cracker pun needs to be short, he says.

"They must also be bad jokes, jokes that cause us to moan," he continues.

The increasingly "awful" the joke, he states the better.

"This is because if no-one laughs – it's the gag's shortcoming, not yours.

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

"It creates a common moment at the gathering and I believe it's lovely."