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

This one-liner is greeted with groans that echo through a warehouse in the capital.

We're at a joke-testing session with a firm that produces products for social events. Its repertoire includes Christmas crackers.

The firm's owner smiles, nearly sheepishly at the gag. But the joke has been selected and will feature in upcoming 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 secret to a great Christmas cracker joke is not the identical as a good joke per se. It is entirely about the setting - in this case, the shared laughter of the Christmas meal with grandparents, children and possibly friends.

"The goal is for the gag to be something that brings the eight-year-old together with the 80-year-old," she adds.

The Neuroscience Of Shared Laughter

Gathering to experience communal laughter is not only ancient, scientists argue, it is likely to be pre-human.

"Therefore when you are laughing with others at the Christmas table you are engaging in what's very likely a really ancient mammalian social sound," explains a neuroscience expert.

Shared amusement, she says, helps forge and strengthen social bonds between people.

Scientists have found that a lack of such social exchanges can significantly damage mental and physical well-being.

"Those you converse with, and share laughter with, it results in enhanced levels of 'happy chemical' uptake," the professor adds.

These natural chemicals are the brain's "happy chemicals" and are released both to reduce tension and discomfort and in response to pleasurable experiences, such as laughing with friends over a truly terrible Christmas cracker joke.

"You're not just chuckling at a silly pun with a Christmas cracker," she states. "You are actually doing a lot of the truly vital task of making, maintaining the connections you have with the people you love."

Which Happens In the Brain?

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

A tremendous amount occurs in response to comedy, it turns out.

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

The research entails scanning the brains of healthy subjects and then exposing them to a database of humorous phrases, paired with either a non-emotional sound, or recorded laughter.

"During the study we got a very fascinating pattern of neural activity," says the professor.

A gag stimulates not just the parts of the brain in charge of auditory processing and interpreting language, but also brain areas involved in both planning and initiating movement and those linked to sight and memory.

Combine these elements as a whole, and individuals listening to a pun have a complex set of brain responses that underpin the laughter we experience.

The Infectious Power of Chuckles

Researchers found that when a funny phrase is paired with chuckles there is a stronger reaction in the mind than the identical word when accompanied by a neutral sound.

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

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

Amusement, says the professor, can be infectious.

So what does this imply for the chuckles found around a Christmas table?

"You laugh more when you are familiar with people," she says, "and you laugh further when you like them or love them."

When it comes to festive cracker puns, she says, the positive effect is more probable to be caused not by the gag itself, but from the response to it.

"The laughter is key. The gag is the dreadful Christmas cracker joke, and it's just a reason to laugh as a group."

The Quest for the Ideal Festive Pun

Is it possible to discover the ultimate gag?

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

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

More than 40,000 jokes submitted, with ratings lodged by 350,000 people around the world, he has a clearer understanding than many as to what succeeds and what fails.

The perfect Christmas cracker pun must be short, he explains.

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

The increasingly "terrible" the gag, he states 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 Christmas cracker jokes is that not one person find them funny.

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