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

This quip is met by groans that echo through a warehouse in London.

This describes a joke-testing session with a firm that makes supplies for social events. Its catalogue features festive crackers.

The company's owner smiles, almost apologetically at the gag. But the joke has made the cut and will appear in future crackers.

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

The secret to a good holiday cracker joke is not the same as a good joke per se. It is entirely about the setting - in this case, the communal laughter of the Christmas dinner table with grandparents, children and potentially neighbours.

"The goal is for the joke to be something that brings the child in harmony with the grandparent," she states.

The Science Of Communal Laughter

Coming together to experience shared laughter is not only nothing new, scientists argue, it is probably to be pre-human.

"So when you are laughing with others around the Christmas table you are dropping into what's very likely a really primordial mammalian play vocalisation," says a professor.

Shared laughter, she says, aids in forge and strengthen social bonds between people.

Researchers have discovered that a lack of such social exchanges can seriously harm mental and physical health.

"Those you converse with, and laugh with, it leads to increased levels of endorphin release," the professor continues.

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

"It's not simply chuckling at a foolish pun with a holiday cracker," she says. "You are in fact doing a lot of the truly vital work of building, preserving the connections you have with the people you care about."

What Occurs Inside the Mind?

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

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

Using brain scanning technology, a kind of neural imager which indicates which parts of the mind are working harder, researchers have been able to chart the areas that get more blood flow.

Testing involves scanning the minds of healthy subjects and then subjecting them to a collection of funny words, paired with either a non-emotional sound, or pre-recorded chuckles.

"In the scanner we got a very interesting activation pattern of neural activity," notes the neuroscientist.

A gag stimulates not just the parts of the brain responsible for auditory processing and interpreting speech, but also neural areas associated with both preparation and initiating movement and those linked to vision and memory.

Combine these elements together, and individuals hearing a joke have a sophisticated series of neural responses that support the laughter we experience.

The Contagious Nature of Chuckles

Scientists discovered that when a funny word is combined with laughter there is a greater response in the brain than the same word when followed by a neutral sound.

"This activation occurred in areas of the brain that you would use to contort your expression into a smile or a laugh," the professor says.

It indicates we are not just reacting to funny words, they are responding to the amusement that accompanies them.

Amusement, according to the professor, can be contagious.

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

"People laugh more 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 explains, the feel-good factor is more likely to be triggered not by the joke itself, but from the response to it.

"It's the laughter. The joke is the terrible holiday cracker pun, and it's just a reason to laugh together."

The Quest for the Perfect Cracker Joke

Is it possible to find the perfect gag?

Probably not, but that has not stopped researchers from attempting to.

In 2001, a professor established a research search for the planet's funniest gag.

Over tens of thousands of jokes later, with scores provided by 350,000 people globally, he has a clearer understanding than many as to what works and what fails.

The perfect Christmas cracker pun must be brief, he explains.

"They must also need to be poor gags, puns that make us groan," he adds.

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

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

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

"It creates a common moment at the gathering and I believe it's wonderful."