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

This one-liner is greeted with groans that resonate through a storage facility in the capital.

This describes a humor-evaluation session with a firm that produces supplies for gatherings. Its catalogue features Christmas crackers.

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

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

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

"The goal is for the gag to be a thing that unites the child in harmony with the grandparent," she states.

The Science Behind Shared Amusement

Coming together to enjoy communal amusement is not only nothing new, scientists argue, it is probably to be older than humanity.

"Therefore when you are chuckling with others around the holiday dinner you are engaging in what's very likely a truly primordial mammalian play sound," explains a neuroscience expert.

Shared laughter, she says, helps make and maintain social connections between individuals.

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

"The people you talk to, and laugh with, it results in increased amounts of endorphin release," the professor adds.

These natural chemicals are the body's "happy chemicals" and are produced both to alleviate stress and pain and in reaction to pleasurable activities, such as laughing with loved ones over a truly awful Christmas cracker joke.

"It's not simply chuckling at a silly pun with a holiday cracker," the expert says. "You are actually doing a lot of the truly vital work of building, preserving the connections you have with the people you love."

What Happens Inside the Mind?

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

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

Using functional magnetic resonance imaging (fMRI), a type of neural imager which shows which parts of the brain are working harder, scientists have been able to map the regions that get more blood flow.

The research entails imaging the minds of volunteer subjects and then exposing them to a collection of humorous phrases, paired with either a neutral sound, or pre-recorded laughter.

"In the scanner we got a very fascinating pattern of neural activity," notes the neuroscientist.

A gag stimulates not just the areas of the brain responsible for hearing and understanding speech, but also brain regions involved in both planning and initiating movement and those linked to vision and memory.

Combine all of this together, and people hearing a joke have a sophisticated set of neural responses that support the amusement we hear.

The Contagious Power of Laughter

Scientists found that when a humorous phrase is combined with chuckles there is a stronger response in the mind than the same phrase when accompanied by a neutral sound.

"This activation occurred in parts of the brain that you would use to contort your face into a grin or a chuckle," she says.

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

Amusement, says the expert, can be infectious.

So what does this mean for the chuckles found at a holiday table?

"You laugh more when you know people," she notes, "and laughter increases further when you are fond of them or love them."

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

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

The Search for the Ideal Cracker Joke

Will we ever find the perfect joke?

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

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

More than 40,000 gags submitted, with ratings lodged by hundreds of thousands of people globally, he has a better understanding than most as to what succeeds and what does not.

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

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

The increasingly "terrible" the gag, he states the more effective.

"The reason is that if nobody finds it funny – it's the joke's fault, not yours.

"What's interesting about the holiday cracker puns is that none of us find them humorous.

"It creates a common moment at the table and I think it's lovely."