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"What was the price did Father Christmas's sled cost? Zero, it was on the house."

This quip is greeted with groans that echo through a storage facility in the capital.

We're at a joke-testing meeting with a firm that produces products for gatherings. Its catalogue includes festive crackers.

The firm's founder grins, almost apologetically at the gag. But the joke has been selected and will feature in future crackers.

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

The secret to a great holiday cracker joke is not the identical as a stand-up gag per se. It is all about the setting - in this instance, the communal amusement of the Christmas meal with elders, kids and possibly neighbours.

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

The Neuroscience Behind Shared Laughter

Gathering to enjoy shared laughter is not only ancient, scientists say, it is probably to be pre-human.

"So when you are chuckling with others around the Christmas table you are engaging in what's almost certainly a truly primordial mammalian play sound," explains a neuroscience expert.

Communal laughter, she says, helps make and maintain social bonds between people.

Scientists have discovered that a absence of these social exchanges can seriously harm mental and physical health.

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

Endorphins are the brain's "happy chemicals" and are produced both to reduce tension and discomfort and in response to pleasurable experiences, such as chuckling with friends over a particularly awful festive cracker joke.

"It's not simply chuckling at a silly pun with a holiday cracker," the expert states. "You are in fact performing a lot of the really vital task of building, preserving the connections you have with those you love."

What Happens In the Brain?

But what is truly happening within the brain when we listen to a gag?

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

Employing functional magnetic resonance imaging (fMRI), a kind of neural imager which indicates which areas of the brain are working harder, researchers have been able to map the areas that get more blood flow.

Testing involves scanning the minds of volunteer subjects and then subjecting them to a database of funny words, accompanied by either a neutral sound, or pre-recorded laughter.

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

A joke activates not just the parts of the brain in charge of hearing and interpreting language, but also brain areas involved in both planning and starting motion and those involved in sight and memory.

Put all of this together, and people listening to a joke have a complex series of neural reactions that underpin the amusement we hear.

The Infectious Power of Chuckles

Scientists discovered that when a funny word is paired with laughter there is a stronger response in the mind than the same phrase when followed by a non-emotional sound.

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

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

Laughter, says the expert, can be contagious.

So what does this imply for the laughter found at a holiday gathering?

"You laugh harder when you know people," she notes, "and you laugh further when you like them or love them."

When it comes to Christmas cracker puns, she says, the feel-good effect is more likely to be caused not by the joke in itself, but from the reaction to it.

"The laughter is key. The gag is the terrible holiday cracker pun, and it's just a reason to chuckle as a group."

The Quest for the Perfect Cracker Joke

Is it possible to discover the ultimate joke?

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

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

Over 40,000 gags submitted, with ratings lodged by hundreds of thousands of participants around the world, he has a better understanding than most as to what works and what fails.

The perfect festive cracker joke needs to be short, he says.

"But they also be poor gags, puns that cause us to moan," he continues.

The increasingly "terrible" the joke, he says the better.

"This is because if nobody laughs – it's the gag's shortcoming, not your own.

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

"That's a common experience at the table and I think it's lovely."