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

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

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

The firm's owner grins, almost apologetically at the joke. But the pun has been selected and will feature in future crackers.

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

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

"The goal is for the joke to be a thing that unites the eight-year-old together with the 80-year-old," she states.

The Science Behind Communal Laughter

Gathering to experience shared amusement is not only nothing new, experts argue, it is probably to be older than humanity.

"So when you are chuckling with others around the Christmas table you are engaging in what's very likely a really ancient mammalian play sound," says a neuroscience expert.

Shared laughter, she explains, helps make and maintain social bonds between individuals.

Scientists have discovered that a lack of such social exchanges can seriously harm both psychological and bodily health.

"Those you talk to, and share laughter with, it results in enhanced levels of 'happy chemical' uptake," the professor continues.

These natural chemicals are the body's "happy chemicals" and are released both to alleviate stress and pain and in reaction to pleasurable experiences, such as chuckling with loved ones over a particularly terrible Christmas cracker gag.

"You're not just laughing at a foolish joke with a holiday cracker," the expert says. "You are actually performing a lot of the truly important task of making, maintaining the connections you have with those you care about."

What Happens In the Brain?

But what is truly happening inside the brain when we listen to a gag?

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

Using brain scanning technology, a kind of brain scanner which indicates which parts of the brain are more active, researchers have been able to map the regions that receive more blood.

Testing entails imaging the minds of healthy subjects and then subjecting them to a collection of humorous words, paired with either a non-emotional sound, or recorded chuckles.

"In the scanner we observed a very interesting activation pattern of neural activity," says the professor.

A joke stimulates not just the parts of the mind in charge of auditory processing and understanding speech, but also neural regions involved in both planning and starting motion and those involved in vision and recall.

Put all of this together, and people listening to a pun have a complex series of brain reactions that support the laughter we experience.

The Contagious Power of Laughter

Researchers found that when a humorous word is paired with laughter there is a stronger reaction in the brain than the identical phrase when accompanied by a non-emotional sound.

"This was in parts of the mind that you would use to move your expression into a smile or a chuckle," the professor explains.

It means people are not just reacting to funny 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?

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

When it comes to Christmas cracker puns, she says, the feel-good effect is more probable to be triggered not by the joke in itself, but from the reaction to it.

"It's the laughter. The gag is the dreadful holiday cracker joke, and it's just a reason to laugh as a group."

The Quest for the Ideal Festive Pun

Will we ever discover the ultimate gag?

Probably not, but that has not prevented researchers from trying to.

Years ago, a psychologist established a scientific search for the world's most humorous gag.

More than tens of thousands of gags submitted, with ratings provided by 350,000 participants globally, he has a clearer idea than many as to what succeeds and what fails.

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

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

The increasingly "terrible" the joke, he says the better.

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

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

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