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

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

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

The firm's founder grins, nearly apologetically at the joke. But the pun has been selected and will appear in upcoming crackers.

"You measure the gag by the volume of moans and the intensity of the groans at the table," she explains.

The key to a good Christmas cracker joke is not the identical as a stand-up gag in itself. It is entirely about the setting - in this instance, the communal laughter of the holiday dinner table with grandparents, children and possibly friends.

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

The Science Of Shared Amusement

Coming together to experience shared amusement is not only nothing new, experts say, it is probably to be pre-human.

"Therefore when you are chuckling with people at the holiday dinner you are dropping into what's almost certainly a really primordial mammalian social sound," says a professor.

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

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

"Those you talk to, and laugh with, it results in increased amounts of 'happy chemical' release," she continues.

These natural chemicals are the body's "happy chemicals" and are produced both to reduce tension and discomfort and in reaction to pleasurable activities, such as chuckling with loved ones over a particularly awful Christmas cracker gag.

"You're not just chuckling at a foolish joke with a holiday cracker," she says. "You are in fact doing a lot of the truly vital work of building, preserving the social bonds you have with those you care about."

What Happens Inside the Brain?

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

A tremendous amount happens in reaction to comedy, it turns out.

Using functional magnetic resonance imaging (fMRI), a kind of brain scanner which shows which parts of the mind are working harder, researchers have been able to chart the areas that receive more blood.

Testing involves scanning the minds of healthy subjects and then subjecting them to a database of humorous words, paired with either a neutral sound, or recorded laughter.

"During the study we got a really fascinating pattern of neural activity," says the professor.

A gag stimulates not just the areas of the brain in charge of hearing and interpreting speech, but also neural regions associated with both preparation and starting motion and those linked to vision and memory.

Put these elements as a whole, and people hearing a pun have a complex series of neural reactions that underpin the amusement we experience.

The Infectious Power of Chuckles

Scientists discovered that when a humorous word is combined with laughter there is a stronger reaction in the brain than the same phrase when followed by a non-emotional sound.

"This was in parts of the brain that you would employ to contort your face into a smile or a laugh," the professor explains.

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

Amusement, says the professor, can be contagious.

So what does this mean for the chuckles heard at a holiday table?

"People laugh harder when you are familiar with others," she notes, "and you laugh further when you are fond of them or love them."

When it comes to festive cracker jokes, she explains, the positive factor is more probable to be caused not by the joke itself, but from the reaction to it.

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

The Quest for the Ideal Festive Pun

Will we ever find the ultimate joke?

Likely not, but that has not stopped experts from attempting to.

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

Over 40,000 gags submitted, with ratings lodged by 350,000 people globally, he has a clearer idea than many as to what succeeds and what fails.

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

"But they also be bad gags, puns that make us groan," he continues.

The more "awful" the gag, he states the better.

"This is because if nobody laughs – it's the joke's shortcoming, not your own.

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

"It creates a shared moment around the gathering and I believe it's wonderful."