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

This quip is greeted with groans that resonate through a warehouse in London.

This describes a joke-testing session with a firm that produces supplies for gatherings. Its catalogue includes Christmas crackers.

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

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

The key to a good Christmas cracker joke is not the identical as a stand-up gag per se. It is entirely about the setting - in this case, the shared amusement of the holiday meal with grandparents, kids and potentially friends.

"You want the gag to be something that brings the child together with the 80-year-old," she states.

The Neuroscience Behind Shared Laughter

Gathering to experience communal laughter is not only ancient, scientists argue, it is probably to be pre-human.

"So when you are chuckling with people around the Christmas table you are engaging in what's almost certainly a really ancient mammalian social sound," explains a professor.

Communal laughter, she explains, aids in forge and strengthen social bonds between people.

Scientists have discovered that a lack of these interactions can seriously damage mental and physical well-being.

"The people you converse with, and laugh with, it leads to increased levels of 'happy chemical' release," the professor adds.

Endorphins are the brain's "happy chemicals" and are released both to alleviate tension and discomfort and in reaction to enjoyable activities, such as laughing with loved ones over a particularly terrible Christmas cracker joke.

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

Which Happens In the Brain?

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

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

Employing functional magnetic resonance imaging (fMRI), a kind of brain scanner which shows which parts of the brain are working harder, researchers have been able to map the regions that get more blood.

The research involves imaging the minds of healthy participants and then exposing them to a database of funny words, paired with either a neutral sound, or pre-recorded chuckles.

"In the scanner we observed a very fascinating pattern of activation," notes the neuroscientist.

A joke stimulates not just the parts of the mind responsible for hearing and understanding language, but also neural regions involved in both preparation and initiating movement and those linked to vision and recall.

Combine these elements as a whole, and people hearing a joke have a sophisticated set of brain reactions that support the laughter we hear.

The Infectious Power of Chuckles

Scientists found that when a funny word is combined with chuckles there is a greater reaction in the mind than the identical word when followed by a neutral sound.

"This activation occurred in parts of the mind that you would employ to move your face into a grin or a chuckle," she explains.

It means we 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 laughter heard at a holiday table?

"People laugh harder when you know people," she says, "and you laugh 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 probable to be triggered not by the joke itself, but from the response to it.

"The laughter is key. The gag is the dreadful Christmas cracker joke, and it's just a pretext to chuckle as a group."

The Quest for the Ideal Cracker Joke

Will we ever discover the ultimate gag?

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

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

Over tens of thousands of gags later, with ratings lodged by hundreds of thousands of people around the world, he has a better idea than most as to what succeeds and what fails.

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

"But they also need to be poor jokes, jokes that cause us to groan," he continues.

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

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

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

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