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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 echo through a warehouse in London.

This describes a joke-testing session with a company that makes supplies for social events. Its repertoire includes Christmas crackers.

The firm's founder grins, nearly apologetically at the joke. But the pun has been selected and will feature in upcoming crackers.

"The success is gauged by the joke by the volume of groans and the intensity of the groans at the table," the founder says.

The secret to a great Christmas cracker pun is not the same as a stand-up gag per se. It is all about the context - in this instance, the communal laughter of the holiday dinner table with grandparents, kids and possibly neighbours.

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

The Science Behind Communal Amusement

Gathering to enjoy shared amusement is not only ancient, scientists argue, it is probably to be older than humanity.

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

Communal laughter, she explains, aids in make and maintain social bonds between people.

Researchers have discovered that a lack of these interactions can significantly damage mental and physical health.

"The people you talk to, and share laughter with, it results in enhanced levels of 'happy chemical' release," the professor adds.

Endorphins are the body's "feel-good compounds" and are produced both to alleviate tension and discomfort and in reaction to enjoyable experiences, such as laughing with friends over a truly awful festive cracker gag.

"It's not simply chuckling at a silly joke with a Christmas cracker," the expert states. "You are in fact doing a lot of the truly important work of making, maintaining the social bonds you have with those you care about."

What Occurs In the Mind?

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

A tremendous amount happens in response to humour, it transpires.

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

Testing entails scanning the brains of volunteer subjects and then subjecting them to a database of funny phrases, accompanied by either a neutral sound, or recorded chuckles.

"During the study we got a really fascinating pattern of neural activity," notes the neuroscientist.

A gag activates not just the parts of the brain in charge of hearing and interpreting language, but also neural regions involved in both preparation and starting movement and those linked to sight and recall.

Put all of this together, and people hearing a joke have a complex set of neural reactions that underpin the laughter we experience.

The Contagious Power of Laughter

Scientists discovered that when a funny phrase is paired with chuckles there is a stronger reaction in the mind than the same phrase when followed by a non-emotional sound.

"This was in parts of the brain that you would use to move your face into a smile or a laugh," the professor explains.

It means people are not just reacting to funny words, they are responding to the laughter that follows them.

Laughter, according to the professor, can be contagious.

So what does this imply for the laughter found at a holiday table?

"People laugh harder when you know people," she says, "and laughter increases more when you are fond of them or care for them."

When it comes to Christmas cracker puns, she says, the feel-good factor is more likely to be caused not by the joke in 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 chuckle as a group."

The Quest for the Perfect Festive Pun

Is it possible to find the perfect joke?

Likely not, but that has not prevented researchers from attempting to.

In 2001, a professor set up a scientific project for the planet's most humorous gag.

Over tens of thousands of jokes submitted, with scores lodged by 350,000 participants around the world, he has a better understanding than many as to what works and what fails.

The perfect festive cracker joke must be brief, he explains.

"But they also need to be poor gags, puns that cause us to moan," he continues.

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

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

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

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