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"What was the price did Father Christmas's sled cost? Nothing, it was on the house."

This joke is met by moans that echo through a storage facility in London.

We're at a humor-evaluation session with a firm that produces supplies for social events. Its repertoire features Christmas crackers.

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

"The success is gauged by the gag by the number of moans and the intensity of the groans around the table," the founder explains.

The secret to a great holiday cracker pun is not the same as a good gag in itself. It is all about the setting - in this case, the communal laughter of the holiday dinner table with grandparents, kids and potentially friends.

"You want the gag to be something that brings the eight-year-old in harmony with the 80-year-old," she adds.

The Science Of Communal Laughter

Gathering to enjoy shared amusement is not only nothing new, experts say, it is likely to be pre-human.

"So when you are chuckling with people at the Christmas dinner you are dropping into what's very likely a truly ancient mammalian play sound," says a professor.

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

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

"Those you talk to, and laugh with, it results in increased levels of 'happy chemical' uptake," the professor adds.

Endorphins are the brain's "happy chemicals" and are released both to alleviate stress and pain and in response to pleasurable experiences, such as laughing with loved ones over a particularly awful festive cracker gag.

"You're not just laughing at a silly joke with a Christmas cracker," the expert says. "You are in fact performing a lot of the truly important work of building, preserving the connections you have with the people you care about."

What Happens Inside the Brain?

But what is truly taking place inside the mind when we listen to a gag?

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

Employing functional magnetic resonance imaging (fMRI), a type of brain scanner which shows which parts of the brain are more active, scientists have been able to chart the areas that get more blood.

Testing involves scanning the brains of healthy participants and then subjecting them to a collection of humorous phrases, accompanied by either a neutral sound, or recorded chuckles.

"During the study we got a really fascinating pattern of activation," notes the professor.

A joke stimulates not just the areas of the mind in charge of auditory processing and understanding speech, but also brain regions associated with both preparation and initiating motion and those involved in sight and recall.

Put all of this as a whole, and people listening to a pun have a sophisticated series of brain responses that underpin the laughter we experience.

The Infectious Nature of Laughter

Scientists found that when a humorous phrase is combined with laughter there is a stronger response in the brain 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 expression into a smile or a chuckle," the professor explains.

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

Amusement, says the expert, can be contagious.

So what does this mean for the laughter heard at a holiday table?

"You laugh harder when you are familiar with others," she notes, "and you laugh further when you are fond of them or care for them."

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

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

The Search for the Perfect Cracker Joke

Will we ever discover the ultimate joke?

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

In 2001, a psychologist established a scientific project for the planet's funniest joke.

More than 40,000 jokes submitted, with scores lodged by 350,000 people around the world, he has a clearer idea than most as to what works and what does not.

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

"But they also be poor jokes, jokes that make us groan," he adds.

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

"This is because 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 considers them humorous.

"It creates a common experience at the gathering and I believe it's wonderful."