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

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

This describes a humor-evaluation meeting with a company that makes products for social events. Its repertoire features Christmas crackers.

The firm's founder smiles, nearly sheepishly at the joke. But the joke has been selected and will appear in future crackers.

"You measure the joke by the volume of moans and the intensity of the groans around the table," the founder says.

The secret to a good holiday cracker pun is not the same as a stand-up joke per se. It is entirely about the context - in this instance, the communal laughter of the holiday dinner table with grandparents, children and potentially friends.

"The goal is for the joke to be something that unites the child together with the 80-year-old," she adds.

The Neuroscience Behind Shared Amusement

Gathering to enjoy communal laughter is not only nothing new, experts say, it is likely to be older than humanity.

"So when you are chuckling with others around the holiday table you are dropping into what's very likely a truly primordial mammalian play sound," says a neuroscience expert.

Shared laughter, she explains, aids in forge and strengthen social connections between people.

Researchers have discovered that a absence of these social exchanges can seriously damage both psychological and bodily health.

"Those you converse with, and laugh with, it leads to enhanced levels of endorphin uptake," she adds.

These natural chemicals are the brain's "feel-good compounds" and are released both to alleviate tension and discomfort and in reaction to enjoyable activities, such as laughing with loved ones over a particularly awful festive cracker gag.

"It's not simply laughing at a silly pun with a holiday cracker," the expert states. "You are actually performing a lot of the truly important work of making, maintaining the connections you have with those you love."

Which Occurs In the Brain?

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

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

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

Testing involves imaging the minds of healthy subjects and then exposing them to a collection of funny words, paired with either a non-emotional sound, or pre-recorded laughter.

"In the scanner we observed a very interesting pattern of neural activity," notes the professor.

A joke activates not just the parts of the brain in charge of auditory processing and understanding speech, but also brain regions associated with both planning and starting movement and those linked to vision and memory.

Combine all of this together, and individuals hearing a pun have a complex set of neural responses that underpin the laughter we hear.

The Contagious Power of Chuckles

Researchers found that when a humorous phrase is combined with chuckles there is a stronger reaction in the mind than the same word when accompanied by a non-emotional sound.

"This was in areas of the mind that you would use to move your face into a smile or a chuckle," the professor explains.

It means we are not just reacting to humorous jokes, they are reacting to the laughter that accompanies them.

Laughter, says the professor, can be contagious.

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

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

When it comes to festive cracker puns, she says, the positive effect 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 laugh as a group."

The Search 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 professor set up a scientific project for the planet's most humorous gag.

Over tens of thousands of gags submitted, with ratings lodged by 350,000 people around the world, he has a clearer understanding than most as to what succeeds and what does not.

The perfect Christmas cracker joke needs to be brief, he says.

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

The increasingly "terrible" the joke, he says the better.

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

"What's interesting about the Christmas cracker jokes is that not one person considers them funny.

"That's a common experience around the table and I think it's lovely."