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

This joke is met by moans that echo through a warehouse in the capital.

This describes a humor-evaluation session with a firm that makes supplies for social events. Its catalogue features festive crackers.

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

"You measure the joke by the number of groans and the intensity of the groans around the table," she explains.

The key to a great Christmas cracker joke is not the same as a stand-up joke in itself. It is entirely about the setting - in this case, the communal amusement of the holiday meal with elders, children and potentially neighbours.

"You want the joke to be something that brings the eight-year-old together with the 80-year-old," she states.

The Neuroscience Of Shared Amusement

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

"So when you are chuckling with others around the holiday dinner you are dropping into what's almost certainly a really primordial mammal social vocalisation," says a neuroscience expert.

Communal laughter, she explains, helps make and maintain social connections between individuals.

Scientists have discovered that a absence of such social exchanges can significantly damage mental and physical health.

"Those you converse with, and laugh with, it results in enhanced amounts of endorphin uptake," she adds.

These natural chemicals are the body'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 particularly awful Christmas cracker gag.

"You're not just laughing at a silly joke with a Christmas cracker," she says. "You are actually doing a lot of the really important task of making, maintaining the connections you have with those you care about."

What Occurs Inside the Mind?

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

A tremendous amount happens in reaction to comedy, it turns out.

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

Testing involves scanning the brains of healthy participants and then exposing them to a database of funny words, paired with either a non-emotional sound, or recorded laughter.

"In the scanner we got a really interesting 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 language, but also brain areas involved in both planning and initiating motion and those linked to vision and recall.

Combine these elements together, and individuals hearing a pun have a complex set of brain reactions that support the laughter we experience.

The Infectious Power of Chuckles

Researchers discovered that when a humorous phrase is paired with chuckles there is a greater reaction in the mind than the same phrase when followed by a neutral sound.

"This activation occurred in parts of the mind that you would use to move your expression into a grin or a laugh," the professor says.

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

Laughter, according to the professor, can be contagious.

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

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

When it comes to Christmas cracker jokes, she explains, the positive effect is more probable to be triggered not by the joke itself, but from the reaction to it.

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

The Quest for the Perfect Festive Pun

Will we ever find the ultimate joke?

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

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

Over 40,000 jokes later, with scores provided by 350,000 people globally, he has a better idea than many as to what works and what does not.

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

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

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

"The reason is that if no-one finds it funny – it's the joke's shortcoming, not your own.

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

"It creates a shared experience around the table and I believe it's wonderful."