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

This one-liner is met by moans that resonate through a storage facility in the capital.

We're at a joke-testing meeting with a firm that produces products for social events. Its repertoire features festive crackers.

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

"You measure the gag by the volume of groans and the intensity of the groans at the table," the founder explains.

The key to a good holiday cracker pun is not the same as a good joke per se. It is all about the setting - in this instance, the shared laughter of the Christmas dinner table with elders, children and possibly friends.

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

The Neuroscience Behind Shared Laughter

Coming together to enjoy communal amusement is not only ancient, scientists argue, it is probably to be pre-human.

"Therefore when you are chuckling with others at the holiday table you are dropping into what's almost certainly a truly primordial mammal play sound," explains a neuroscience expert.

Shared laughter, she explains, helps forge and strengthen social connections between individuals.

Researchers have found that a lack of these interactions can significantly damage mental and physical well-being.

"The people you converse with, and share laughter with, it results in increased amounts of 'happy chemical' release," she continues.

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

"It's not simply laughing at a foolish pun with a Christmas cracker," she states. "You are in fact performing a lot of the really vital work of making, maintaining the social bonds you have with the people you love."

Which Happens Inside the Mind?

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

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

Using functional magnetic resonance imaging (fMRI), a kind of brain scanner which indicates which areas of the mind are working harder, scientists have been able to chart the areas that get more blood flow.

The research entails imaging the minds of healthy subjects and then subjecting them to a database of humorous phrases, paired with either a non-emotional sound, or pre-recorded chuckles.

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

A joke stimulates not just the parts of the brain in charge of auditory processing and interpreting speech, but also neural regions associated with both planning and initiating motion and those linked to vision and memory.

Combine these elements together, and people listening to a joke have a sophisticated series of neural responses that underpin the laughter we experience.

The Contagious Nature of Laughter

Researchers found that when a humorous phrase is combined with laughter there is a greater response in the brain than the identical word when accompanied by a non-emotional sound.

"This was in areas of the brain that you would use to move your face into a smile or a laugh," the professor says.

It indicates people are not just responding to funny jokes, they are responding to the laughter that follows them.

Laughter, says the professor, can be contagious.

So what does this imply for the laughter heard around a holiday gathering?

"You laugh harder when you know people," she notes, "and laughter increases more when you like them or care for them."

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

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

The Search for the Ideal Cracker Joke

Is it possible to find the ultimate joke?

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

In 2001, a professor established a research search for the world's most humorous joke.

Over tens of thousands of gags submitted, with scores lodged by hundreds of thousands of people globally, he has a better understanding than most as to what succeeds and what does not.

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

"They must also need to be poor jokes, puns that cause us to moan," he continues.

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

"The reason is that if no-one laughs – it's the gag's fault, not your own.

"The fascinating part about the Christmas cracker puns is that not one person find them funny.

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