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

This quip is greeted with groans that echo through a storage facility in the capital.

We're at a joke-testing session with a company that makes products for social events. Its repertoire includes festive crackers.

The firm's founder smiles, almost sheepishly at the gag. But the joke has been selected and will feature in upcoming crackers.

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

The secret to a great holiday cracker joke is not the identical as a good joke per se. It is entirely about the setting - in this instance, the communal laughter of the holiday dinner table with elders, kids and possibly neighbours.

"You want the joke to be something that unites the eight-year-old in harmony with the grandparent," she states.

The Science Of Shared Amusement

Gathering to enjoy shared laughter is not only ancient, scientists say, it is likely to be pre-human.

"So when you are chuckling with people around the holiday dinner you are dropping into what's almost certainly a truly ancient mammalian play vocalisation," explains a neuroscience expert.

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

Researchers have discovered that a lack of such social exchanges can seriously damage both psychological and bodily well-being.

"Those you converse with, and laugh with, it results in increased levels of 'happy chemical' uptake," she continues.

These natural chemicals are the brain's "feel-good compounds" and are produced both to reduce tension and discomfort and in reaction to enjoyable experiences, such as laughing with friends over a particularly terrible Christmas cracker gag.

"You're not just laughing at a foolish pun with a holiday cracker," the expert states. "You are actually doing a lot of the truly vital work of making, maintaining the connections you have with the people you care about."

Which Happens In the Mind?

But what is truly happening within the mind when we hear a joke?

An awful lot happens in response to humour, it transpires.

Employing brain scanning technology, a type of neural imager which shows which parts of the mind are more active, scientists have been able to map the regions that receive more blood.

Testing entails imaging the brains of healthy subjects and then exposing them to a database of humorous phrases, paired with either a neutral sound, or pre-recorded chuckles.

"In the scanner we got a really interesting activation pattern of activation," notes the neuroscientist.

A joke stimulates not just the parts of the brain in charge of hearing and interpreting language, but also neural regions associated with both preparation and starting motion and those linked to sight and memory.

Combine these elements as a whole, and individuals listening to a joke have a complex series of neural responses that support the laughter we hear.

The Infectious Nature of Chuckles

Researchers found that when a funny phrase is paired with laughter there is a stronger response in the mind than the identical word when accompanied by a neutral sound.

"This was in areas of the brain that you would use to move your face into a grin or a laugh," she says.

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

Amusement, says the expert, can be infectious.

So what does this imply for the chuckles heard at a holiday table?

"People laugh harder when you are familiar with people," she notes, "and you laugh further when you like them or love them."

When it comes to Christmas cracker jokes, she explains, the feel-good effect is more likely to be caused not by the gag 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 as a group."

The Search for the Perfect Cracker Joke

Is it possible to find the perfect joke?

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

Years ago, a psychologist established a scientific project for the world's most humorous joke.

Over 40,000 jokes later, with ratings lodged by 350,000 participants globally, he has a better idea than most as to what works and what does not.

The ideal festive cracker pun needs to be short, he says.

"But they also be bad gags, jokes that make us groan," he continues.

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

"This is because if nobody finds it funny – it's the gag's fault, not your own.

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

"That's a common experience around the table and I think it's wonderful."