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

This one-liner is greeted with moans that resonate through a storage facility in London.

We're at a humor-evaluation meeting with a company that makes supplies for gatherings. Its repertoire includes festive crackers.

The firm's founder smiles, almost sheepishly at the gag. But the pun has made the cut and will feature in future crackers.

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

The secret to a great holiday cracker joke is not the same as a good gag per se. It is entirely about the context - in this case, the shared amusement of the holiday dinner table with grandparents, kids and possibly neighbours.

"The goal is for the gag to be a thing that brings the child in harmony with the grandparent," she states.

The Neuroscience Behind Shared Amusement

Coming together to experience shared laughter is not only ancient, scientists say, it is likely to be pre-human.

"Therefore when you are laughing with others at the holiday dinner you are dropping into what's almost certainly a truly primordial mammalian play vocalisation," explains a neuroscience expert.

Shared laughter, she explains, aids in make and maintain social bonds between people.

Scientists have found that a absence of such interactions can significantly damage mental and physical health.

"The people you converse with, and laugh with, it results in enhanced levels of 'happy chemical' uptake," the professor continues.

These natural chemicals are the body's "happy chemicals" and are produced both to alleviate tension and discomfort and in response to pleasurable activities, such as laughing with friends over a particularly awful festive cracker gag.

"It's not simply laughing at a silly joke with a Christmas cracker," the expert says. "You are actually doing a lot of the truly vital work of building, preserving the social bonds you have with the people you care about."

Which Occurs In the Brain?

But what is actually happening within the brain when we hear a joke?

A tremendous amount occurs in response to comedy, it turns out.

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

Testing involves imaging the brains of volunteer participants and then subjecting them to a database of humorous words, paired with either a non-emotional sound, or recorded laughter.

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

A joke activates not just the parts of the mind in charge of auditory processing and interpreting speech, but also brain regions associated with both preparation and starting motion and those linked to sight and memory.

Combine all of this together, and people listening to a joke have a sophisticated series of brain reactions that underpin the laughter we hear.

The Contagious Nature of Chuckles

Researchers discovered that when a funny word is combined with chuckles there is a stronger response in the mind than the same phrase when accompanied by a neutral sound.

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

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

Amusement, says the expert, can be infectious.

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

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

When it comes to festive cracker jokes, she says, the feel-good factor is more probable to be triggered not by the joke in itself, but from the reaction to it.

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

The Quest for the Perfect Festive Pun

Is it possible to discover the perfect joke?

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

In 2001, a professor set up a research search for the planet's funniest gag.

More than 40,000 jokes later, with scores provided by hundreds of thousands of people globally, he has a clearer understanding than many as to what works and what fails.

The ideal festive cracker joke must be short, he says.

"But they also be poor gags, puns that cause us to moan," he continues.

The more "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 Christmas cracker puns is that none of us find them humorous.

"It creates a shared experience around the table and I believe it's lovely."