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

This joke is met by groans that resonate through a storage facility in London.

This describes a joke-testing session with a firm that makes supplies for gatherings. Its catalogue features Christmas crackers.

The company's founder smiles, almost apologetically at the joke. But the pun has made the cut and will appear in future crackers.

"You measure the gag by the volume of groans and the loudness of the groans around the table," she explains.

The key to a good holiday cracker pun is not the identical as a good joke in itself. It is all about the context - in this case, the shared laughter of the holiday dinner table with grandparents, children and possibly neighbours.

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

The Science Behind Communal Amusement

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

"Therefore when you are chuckling with others around the holiday table you are dropping into what's very likely a really primordial mammal play sound," explains a professor.

Communal amusement, she explains, helps make and maintain social bonds between individuals.

Scientists have found that a absence of these interactions can seriously harm both psychological and bodily health.

"Those you converse with, and share laughter with, it results in increased levels of endorphin uptake," she adds.

These natural chemicals are the brain's "happy chemicals" and are released both to reduce tension and discomfort and in reaction to enjoyable activities, such as chuckling with friends over a particularly awful festive cracker joke.

"It's not simply chuckling at a silly pun with a holiday cracker," she states. "You are actually performing a lot of the really important work of making, maintaining the social bonds you have with the people you care about."

What Happens Inside the Brain?

But what is actually happening inside the mind when we hear a joke?

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

Employing functional magnetic resonance imaging (fMRI), a type of neural imager which shows which areas of the brain are more active, researchers have been able to map the regions that get more blood flow.

Testing entails imaging the minds of healthy participants and then subjecting them to a database of humorous words, paired with either a neutral sound, or recorded chuckles.

"During the study we got a very interesting pattern of activation," says the professor.

A gag activates not just the areas of the mind responsible for auditory processing and understanding speech, but also neural regions associated with both preparation and starting motion and those involved in vision and memory.

Put these elements together, and people hearing a joke have a complex set of neural reactions that underpin the laughter we hear.

The Contagious Nature of Chuckles

Scientists discovered that when a funny phrase is paired with chuckles there is a stronger reaction in the mind than the identical word when accompanied by a non-emotional sound.

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

It means we are not just responding to funny words, they are reacting to the laughter that accompanies them.

Laughter, according to the professor, can be contagious.

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

"People laugh more when you are familiar with others," she says, "and laughter increases further when you are fond of them or care for them."

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

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

The Search for the Ideal Cracker Joke

Will we ever find the perfect joke?

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

Years ago, a professor set up a research project for the world's most humorous joke.

Over tens of thousands of jokes submitted, with ratings provided by 350,000 participants around the world, he has a better understanding than most as to what succeeds and what fails.

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

"But they also be bad jokes, jokes that make us moan," he continues.

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

"The reason is that if nobody finds it funny – it's the gag's fault, not your own.

"What's interesting about the holiday cracker puns is that not one person find them humorous.

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