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

This joke is greeted with moans that resonate through a warehouse in the capital.

We're at a joke-testing meeting with a company that produces supplies for social events. Its repertoire includes festive crackers.

The firm's founder smiles, nearly sheepishly at the joke. But the joke has made the cut and will appear in upcoming crackers.

"The success is gauged by the gag by the volume of groans and the intensity of the groans around the table," the founder says.

The key to a good Christmas cracker joke is not the same as a stand-up gag in itself. It is entirely about the setting - in this case, the communal amusement of the Christmas meal with grandparents, kids and potentially friends.

"The goal is for the joke to be something that brings the child together with the grandparent," she states.

The Science Behind Communal Laughter

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

"Therefore when you are laughing with others at the Christmas dinner you are engaging in what's almost certainly a truly primordial mammal social vocalisation," says a professor.

Communal laughter, she says, helps forge and strengthen social connections between people.

Scientists have found that a lack of such social exchanges can seriously harm both psychological and bodily well-being.

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

Endorphins are the brain's "feel-good compounds" and are released both to reduce stress and pain and in response to pleasurable activities, such as chuckling with friends over a truly awful Christmas cracker joke.

"You're not just laughing at a silly joke with a holiday cracker," she states. "You are in fact performing a lot of the truly vital task of building, preserving the social bonds you have with the people you love."

Which Happens In the Brain?

But what is actually happening within the mind when we listen to a joke?

A tremendous amount occurs in response to comedy, it transpires.

Using functional magnetic resonance imaging (fMRI), a kind of neural imager which shows which areas of the brain are working harder, scientists have been able to chart the areas that receive more blood flow.

Testing entails scanning the minds of healthy participants and then exposing them to a collection of funny words, accompanied by either a neutral sound, or recorded laughter.

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

A joke activates not just the areas of the mind responsible for auditory processing and interpreting speech, but also brain areas involved in both planning and initiating motion and those linked to sight and recall.

Combine all of this as a whole, and people listening to a joke have a complex series of neural reactions that underpin the laughter we hear.

The Contagious Power of Chuckles

Researchers discovered that when a humorous word is paired with chuckles there is a greater response in the brain than the same word when accompanied by a neutral sound.

"This activation occurred in parts of the mind that you would use to move your face into a grin or a chuckle," she says.

It indicates people are not just reacting to humorous jokes, they are reacting to the amusement that follows them.

Laughter, says the professor, can be contagious.

So what does this imply for the laughter found around a holiday table?

"You laugh more when you are familiar with people," she says, "and laughter increases further when you are fond of them or care for them."

When it comes to festive cracker jokes, she explains, the feel-good factor is more likely to be caused not by the gag itself, but from the reaction to it.

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

The Search for the Perfect Cracker Joke

Is it possible to find the perfect joke?

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

In 2001, a professor established a scientific project for the world's funniest joke.

More than 40,000 gags submitted, with scores provided by 350,000 participants around the world, he has a better understanding than most as to what works and what does not.

The perfect festive cracker joke must be brief, he explains.

"But they also need to be poor gags, jokes that make us groan," he continues.

The more "awful" the joke, he says the better.

"This is because if no-one laughs – it's the gag's fault, not your own.

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

"It creates a shared experience at the gathering and I believe it's wonderful."