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"What was the price did Santa'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 gatherings. Its catalogue includes festive crackers.

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

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

The secret to a good holiday cracker pun is not the same as a good joke in itself. It is entirely about the context - in this case, the shared amusement of the Christmas meal with grandparents, children and possibly neighbours.

"The goal is for the gag to be something that unites the eight-year-old in harmony with the grandparent," she adds.

The Science Of Shared Laughter

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

"So when you are laughing with people at the holiday dinner you are dropping into what's very likely a really primordial mammal play sound," says a neuroscience expert.

Communal laughter, she says, aids in make and maintain social connections between people.

Researchers have discovered that a absence of such social exchanges can significantly damage both psychological and bodily well-being.

"Those you converse with, and share laughter with, it results in increased amounts of endorphin release," the professor adds.

These natural chemicals are the brain's "happy chemicals" and are released both to alleviate stress and pain and in reaction to pleasurable activities, such as laughing with loved ones over a truly awful Christmas cracker joke.

"It's not simply chuckling at a silly joke with a holiday cracker," she states. "You are actually performing a lot of the truly vital work of building, preserving the connections you have with the people you love."

What Occurs Inside the Brain?

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

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

Using functional magnetic resonance imaging (fMRI), a type of brain scanner which shows which parts of the mind are more active, scientists have been able to map the regions that receive more blood flow.

The research involves imaging the minds of volunteer subjects and then subjecting them to a collection of funny words, paired with either a non-emotional sound, or pre-recorded laughter.

"During the study we observed a very interesting pattern of activation," says the neuroscientist.

A joke stimulates not just the parts of the brain in charge of auditory processing and interpreting language, but also brain regions involved in both planning and starting motion and those linked to sight and recall.

Put all of this as a whole, and people hearing a joke have a sophisticated set of neural reactions that underpin the laughter we experience.

The Infectious Nature of Chuckles

Scientists discovered that when a funny phrase is paired with chuckles there is a stronger response in the brain than the identical word when followed by a neutral sound.

"This was in areas of the brain that you would use to move your expression into a smile or a laugh," she explains.

It indicates people are not just responding to humorous jokes, they are reacting to the amusement that follows them.

Amusement, says the expert, can be contagious.

So what does this mean for the chuckles heard around a Christmas table?

"You laugh harder when you are familiar with others," she says, "and laughter increases further when you like them or love them."

When it comes to Christmas cracker puns, she explains, 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 holiday cracker pun, and it's just a reason to chuckle together."

The Quest for the Ideal Cracker Joke

Will we ever discover the ultimate joke?

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

In 2001, a psychologist set up a research project for the world's funniest joke.

Over tens of thousands of gags submitted, with ratings lodged by hundreds of thousands of participants globally, he has a better understanding than many as to what works and what does not.

The ideal festive cracker pun must be short, he explains.

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

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

"The reason is that if no-one laughs – it's the gag's fault, not your own.

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

"That's a shared experience at the table and I believe it's lovely."