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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 warehouse in London.

We're at a joke-testing session with a firm that produces products for gatherings. Its repertoire features festive crackers.

The company's founder grins, almost apologetically at the joke. But the pun has been selected and will appear in upcoming crackers.

"The success is gauged by the gag by the volume of groans and the loudness of the groans at the table," she says.

The key to a great Christmas cracker joke is not the same as a good joke in itself. It is entirely about the context - in this instance, the communal amusement of the Christmas dinner table with grandparents, kids and potentially neighbours.

"You want the joke to be a thing that brings the eight-year-old together with the grandparent," she states.

The Science Of Communal Amusement

Coming together to experience shared amusement 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 engaging in what's almost certainly a truly primordial mammalian play sound," says a professor.

Shared laughter, she says, aids in make and maintain social bonds between people.

Scientists have found that a absence of such interactions can seriously damage both psychological and bodily health.

"Those you talk to, and laugh with, it results in increased levels of 'happy chemical' release," she continues.

These natural chemicals are the brain's "feel-good compounds" and are released both to alleviate stress and pain and in reaction to pleasurable experiences, such as laughing with friends over a truly terrible Christmas cracker gag.

"You're not just laughing at a foolish pun with a Christmas cracker," she states. "You are actually doing a lot of the truly vital work of making, maintaining the connections you have with those you love."

Which Happens In the Brain?

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

A tremendous amount happens in reaction to comedy, it transpires.

Using brain scanning technology, a type of neural imager which shows which parts of the brain are working harder, scientists have been able to chart the regions that receive more blood flow.

The research entails scanning the brains of healthy subjects and then subjecting them to a collection of funny phrases, paired with either a neutral sound, or pre-recorded laughter.

"During the study we got a very interesting pattern of activation," says the professor.

A gag stimulates not just the parts of the brain responsible for hearing and understanding language, but also brain areas associated with both planning and starting motion and those involved in vision and recall.

Put these elements as a whole, and individuals hearing a pun have a complex series of brain responses that underpin the laughter we experience.

The Contagious Power of Laughter

Researchers discovered that when a funny word is combined with chuckles 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 chuckle," she explains.

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

Amusement, according to the professor, can be infectious.

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

"People laugh more when you are familiar with people," she notes, "and you laugh more when you are fond of them or care for them."

When it comes to Christmas cracker puns, she explains, the positive effect is more likely to be triggered not by the gag itself, but from the reaction to it.

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

The Search for the Ideal Festive Pun

Will we ever find the ultimate gag?

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

In 2001, a psychologist established a scientific project for the world's funniest gag.

Over tens of thousands of jokes submitted, with scores lodged by hundreds of thousands of people globally, he has a better idea than most as to what works and what fails.

The perfect Christmas cracker joke needs to be brief, he explains.

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

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

"The reason is that if nobody finds it funny – it's the gag's fault, not your own.

"The fascinating part about the holiday cracker jokes is that none of us find them humorous.

"It creates a shared moment around the table and I believe it's lovely."