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

This quip is met by moans that echo through a warehouse in London.

This describes a joke-testing meeting with a company that makes supplies for social events. Its repertoire includes festive crackers.

The company's founder grins, almost apologetically at the joke. But the joke has made the cut and will feature in upcoming crackers.

"The success is gauged by the joke by the number of groans and the intensity of the groans around the table," she says.

The secret to a great Christmas cracker joke is not the same as a stand-up joke per se. It is entirely about the context - in this instance, the communal amusement of the holiday dinner table with grandparents, children and possibly neighbours.

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

The Neuroscience Of Shared Amusement

Coming together to enjoy communal amusement is not only nothing new, scientists say, it is likely to be pre-human.

"Therefore when you are chuckling with people at the Christmas dinner you are dropping into what's very likely a really primordial mammal social sound," explains a professor.

Communal amusement, she explains, aids in make and maintain social bonds between individuals.

Researchers have found that a lack of such social exchanges can significantly damage both psychological and bodily health.

"Those you talk to, and laugh with, it leads to increased levels of endorphin release," she adds.

These natural chemicals are the brain's "feel-good compounds" and are produced both to alleviate tension and discomfort and in response to pleasurable experiences, such as chuckling with loved ones over a particularly terrible Christmas cracker gag.

"It's not simply chuckling at a silly pun with a Christmas cracker," she says. "You are actually performing a lot of the truly vital task of building, preserving the connections you have with those you love."

What Occurs Inside the Brain?

But what is truly taking place within the mind when we listen to a joke?

A tremendous amount happens in reaction to comedy, it transpires.

Using functional magnetic resonance imaging (fMRI), a kind of neural imager which shows which parts of the mind are working harder, researchers have been able to chart the regions that get more blood flow.

The research involves scanning the brains of healthy subjects and then exposing them to a database of funny words, accompanied by either a non-emotional sound, or recorded laughter.

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

A joke activates not just the areas of the brain responsible for hearing and understanding language, but also neural regions involved in both preparation and initiating motion and those linked to vision and recall.

Put all of this together, and individuals hearing a pun have a sophisticated series of neural responses that support the amusement we hear.

The Contagious Power of Chuckles

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

"This activation occurred in parts of the mind that you would use to contort your expression into a smile or a chuckle," the professor says.

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

Amusement, according to the professor, can be contagious.

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

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

When it comes to festive cracker puns, she explains, the positive effect is more likely to be triggered not by the gag itself, but from the reaction to it.

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

The Quest for the Ideal Festive Pun

Is it possible to find the ultimate joke?

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

In 2001, a professor set up a scientific search for the world's funniest joke.

Over 40,000 gags later, with scores provided by hundreds of thousands of people around the world, he has a clearer idea than many as to what succeeds and what fails.

The ideal Christmas cracker joke must be short, he explains.

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

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

"The reason is that if nobody finds it funny – it's the joke's shortcoming, not your own.

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

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