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

This quip is greeted with moans that resonate through a warehouse in London.

This describes a joke-testing session with a firm that makes products for gatherings. Its catalogue features festive crackers.

The firm's founder grins, almost sheepishly at the joke. But the joke has made the cut and will appear in future crackers.

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

The secret to a great Christmas cracker pun is not the identical as a stand-up gag in itself. It is entirely about the context - in this instance, the communal laughter of the Christmas dinner table with grandparents, children and potentially friends.

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

The Science Of Shared Amusement

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

"Therefore when you are chuckling with others around the Christmas table you are dropping into what's almost certainly a truly primordial mammalian social vocalisation," explains a neuroscience expert.

Shared laughter, she explains, helps make and maintain social connections between people.

Researchers have discovered that a lack of such social exchanges can significantly harm mental and physical health.

"Those you talk to, and laugh with, it results in enhanced amounts of 'happy chemical' release," the professor adds.

These natural chemicals are the brain's "happy chemicals" and are released both to alleviate tension and discomfort and in response to enjoyable activities, such as chuckling with loved ones over a particularly awful Christmas cracker joke.

"It's not simply chuckling at a foolish joke with a holiday cracker," she says. "You are in fact performing a lot of the really vital work of making, maintaining the social bonds you have with the people you care about."

What Happens Inside the Brain?

But what is truly taking place within the brain when we hear a joke?

An awful lot occurs in reaction to comedy, it turns out.

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

Testing involves imaging the minds of volunteer subjects and then subjecting them to a database of funny words, paired with either a neutral sound, or recorded chuckles.

"In the scanner we observed a very interesting pattern of activation," says the neuroscientist.

A gag activates not just the parts of the brain responsible for auditory processing and interpreting speech, but also neural regions associated with both preparation and starting movement and those linked to vision and memory.

Put all of this together, and individuals listening to a pun have a sophisticated set of neural responses that underpin the laughter we experience.

The Infectious Nature of Laughter

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

"This was in areas of the mind that you would employ to contort your expression into a smile or a laugh," she explains.

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

Laughter, according to the expert, can be infectious.

So what does this mean for the laughter found around a holiday gathering?

"People laugh more when you know others," she says, "and you laugh further when you are fond of them or love them."

When it comes to festive cracker jokes, she says, the feel-good factor is more probable to be triggered not by the joke in itself, but from the response to it.

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

The Quest for the Perfect Cracker Joke

Will we ever find the ultimate joke?

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

In 2001, a psychologist established a scientific search for the world's funniest joke.

Over 40,000 gags later, with ratings lodged by 350,000 people around the world, he has a better understanding than many as to what works and what fails.

The ideal festive cracker pun must be brief, he explains.

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

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

"This is because if nobody laughs – it's the gag's shortcoming, not your own.

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

"That's a shared experience around the gathering and I think it's wonderful."