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"What was the price did Santa's sled cost? Zero, it was on the house."

This quip is greeted with groans that echo through a storage facility in London.

This describes a humor-evaluation session with a firm that makes products for gatherings. Its repertoire includes Christmas crackers.

The company's owner smiles, almost apologetically at the joke. But the pun has made the cut and will feature in upcoming crackers.

"You measure the gag by the number of groans and the loudness of the groans around the table," the founder explains.

The secret to a great Christmas cracker pun is not the same as a stand-up gag per se. It is entirely about the setting - in this instance, the shared amusement of the Christmas dinner table with grandparents, kids and possibly neighbours.

"You want the joke to be a thing that brings the child in harmony with the 80-year-old," she adds.

The Neuroscience Behind Communal Amusement

Coming together to enjoy communal amusement is not only nothing new, scientists argue, it is likely to be older than humanity.

"So when you are chuckling with people around the Christmas table you are engaging in what's almost certainly a really primordial mammalian play vocalisation," says a neuroscience expert.

Shared laughter, she says, helps forge and strengthen social connections between people.

Researchers have found that a lack of these interactions can seriously harm mental and physical well-being.

"Those you converse with, and laugh with, it leads to increased amounts of 'happy chemical' uptake," she continues.

These natural chemicals are the brain's "happy chemicals" and are released both to reduce stress and pain and in response to pleasurable experiences, such as chuckling with loved ones over a particularly awful festive cracker gag.

"You're not just chuckling at a silly joke with a holiday cracker," the expert says. "You are actually performing a lot of the truly vital task of building, preserving the social bonds you have with the people you love."

Which Occurs Inside the Mind?

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

An awful lot happens in response to comedy, it transpires.

Using functional magnetic resonance imaging (fMRI), a type of neural imager which indicates which parts of the mind are more active, researchers have been able to map the areas that get more blood.

The research involves scanning the minds of volunteer subjects and then exposing them to a collection of humorous phrases, accompanied by either a non-emotional sound, or recorded laughter.

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

A joke stimulates not just the areas of the mind responsible for auditory processing and understanding speech, but also neural areas associated with both planning and initiating movement and those involved in vision and recall.

Put these elements together, and individuals listening to a joke have a sophisticated series of neural reactions that support the amusement we hear.

The Contagious Power of Laughter

Scientists found that when a funny phrase is paired with chuckles there is a greater response in the brain than the identical word when accompanied by a neutral sound.

"This activation occurred in parts of the brain that you would use to move your face into a grin or a laugh," she explains.

It indicates people are not just responding to humorous words, they are responding to the laughter that follows them.

Laughter, says the expert, can be contagious.

So what does this imply for the laughter heard at a holiday gathering?

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

When it comes to Christmas cracker jokes, she explains, the positive factor is more probable to be triggered not by the gag itself, but from the reaction to it.

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

The Search for the Ideal Cracker Joke

Is it possible to discover the ultimate joke?

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

In 2001, a professor established a scientific project for the world's funniest joke.

Over tens of thousands of jokes submitted, with ratings lodged by 350,000 people globally, he has a better idea than most as to what succeeds and what fails.

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

"They must also need to be bad jokes, jokes that make us groan," he adds.

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

"This is because if no-one finds it funny – it's the joke's shortcoming, not yours.

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

"That's a common experience at the gathering and I believe it's wonderful."