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

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

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

The company's owner grins, nearly apologetically at the gag. But the pun has made the cut and will feature in future crackers.

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

The secret to a good holiday cracker joke is not the same as a good gag in itself. It is all about the setting - in this instance, the shared amusement of the Christmas meal with grandparents, children and possibly neighbours.

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

The Science Behind Communal Laughter

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

"So when you are laughing with others around the Christmas table you are engaging in what's almost certainly a truly primordial mammalian play vocalisation," says a professor.

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

Scientists have found that a lack of such interactions can significantly harm both psychological and bodily well-being.

"The people you talk to, and share laughter with, it leads to increased amounts of endorphin release," she continues.

These natural chemicals are the brain's "happy chemicals" and are produced both to alleviate stress and pain and in reaction to enjoyable activities, such as chuckling with loved ones over a truly terrible Christmas cracker gag.

"It's not simply laughing at a silly joke with a Christmas cracker," the expert states. "You are actually performing a lot of the truly important work of building, preserving the connections you have with the people you care about."

Which Occurs Inside the Brain?

But what is truly taking place inside the brain when we hear a gag?

A tremendous amount occurs in response to comedy, it turns out.

Using functional magnetic resonance imaging (fMRI), a kind of brain scanner which indicates which parts of the mind are more active, researchers have been able to chart the regions that receive more blood flow.

The research entails scanning the brains of healthy participants and then exposing them to a collection of funny phrases, accompanied by either a non-emotional sound, or recorded laughter.

"During the study we observed a very fascinating activation pattern of neural activity," says the professor.

A joke stimulates not just the areas of the mind in charge of hearing and interpreting speech, but also brain regions involved in both preparation and initiating movement and those involved in sight and recall.

Put these elements together, and people hearing a pun have a sophisticated set of brain responses that underpin the laughter we experience.

The Contagious Power of Chuckles

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

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

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

Amusement, says the professor, can be infectious.

So what does this mean for the chuckles found at a holiday gathering?

"People laugh more when you know people," she notes, "and you laugh further when you like them or care for them."

When it comes to Christmas cracker jokes, she explains, the positive effect 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 dreadful holiday cracker pun, and it's just a reason to chuckle as a group."

The Search for the Perfect Festive Pun

Will we ever find the ultimate gag?

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

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

More than 40,000 jokes submitted, with ratings provided by hundreds of thousands of people globally, he has a clearer understanding than many as to what succeeds and what does not.

The ideal Christmas cracker pun must be brief, he says.

"They must also need to be bad jokes, puns that cause us to groan," he continues.

The increasingly "terrible" the joke, he says the more effective.

"This is because if no-one finds it funny – it's the gag's shortcoming, not your own.

"What's interesting about the holiday cracker jokes is that not one person considers them humorous.

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