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

This quip is greeted with moans that resonate through a storage facility in London.

We're at a humor-evaluation session with a company that makes supplies for social events. Its repertoire includes Christmas crackers.

The firm's founder grins, almost sheepishly at the gag. But the pun has been selected and will feature in future crackers.

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

The secret to a good Christmas cracker pun is not the identical as a stand-up joke per se. It is entirely about the context - in this case, the communal laughter of the holiday dinner table with elders, kids and possibly neighbours.

"You want the gag to be a thing that brings the eight-year-old together with the 80-year-old," she adds.

The Neuroscience Behind Shared Laughter

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

"Therefore when you are chuckling with others around the Christmas dinner you are engaging in what's very likely a truly ancient mammalian social sound," explains a neuroscience expert.

Shared amusement, she explains, helps forge and strengthen social bonds between individuals.

Scientists have discovered that a lack of such interactions can significantly harm mental and physical well-being.

"The people you converse with, and laugh with, it results in increased levels of endorphin uptake," the professor continues.

These natural chemicals are the brain's "happy chemicals" and are produced both to reduce tension and discomfort and in reaction to pleasurable experiences, such as laughing with friends over a particularly terrible festive cracker gag.

"You're not just chuckling at a silly pun with a Christmas cracker," she states. "You are actually performing a lot of the truly important task of making, maintaining the connections you have with the people you care about."

Which Happens Inside the Brain?

But what is actually taking place inside the brain when we listen to a gag?

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

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

Testing entails scanning the brains of volunteer participants and then subjecting them to a collection of funny phrases, paired with either a non-emotional sound, or recorded laughter.

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

A joke activates not just the parts of the mind responsible for auditory processing and understanding language, but also neural areas associated with both preparation and starting motion and those linked to vision and memory.

Combine these elements as a whole, and people hearing a pun have a complex set of neural reactions that underpin the laughter we hear.

The Infectious Nature of Laughter

Researchers discovered that when a funny phrase is combined with laughter there is a stronger response in the brain than the identical word when accompanied by a non-emotional sound.

"This activation occurred in parts of the brain that you would employ to contort your face into a grin or a laugh," she says.

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

Amusement, says the expert, can be contagious.

So what does this mean for the laughter found around a holiday table?

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

When it comes to festive cracker jokes, she says, the feel-good effect is more likely to be caused not by the joke in itself, but from the response to it.

"The laughter is key. The gag is the dreadful Christmas cracker pun, and it's just a reason to laugh together."

The Quest for the Ideal Cracker Joke

Will we ever discover the ultimate gag?

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

Years ago, a psychologist established a scientific search for the world's funniest gag.

Over 40,000 gags later, with ratings lodged by hundreds of thousands of participants around the world, he has a better understanding than many as to what works and what fails.

The perfect festive cracker pun needs to be brief, he explains.

"They must also be poor jokes, jokes that make us moan," he continues.

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

"This is because if no-one laughs – it's the gag's fault, not your own.

"The fascinating part about the Christmas cracker puns is that none of us find them funny.

"It creates a shared moment around the table and I think it's wonderful."