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"How much did Santa's sleigh cost? Zero, it was on the house."

This one-liner is met by moans that resonate through a storage facility in the capital.

We're at a joke-testing session with a firm that produces supplies for gatherings. Its catalogue includes festive crackers.

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

"You measure the joke by the number of groans and the intensity of the groans around the table," the founder explains.

The secret to a good Christmas cracker pun is not the identical as a stand-up joke per se. It is all about the context - in this case, the communal laughter of the holiday meal with grandparents, children and potentially neighbours.

"The goal is for the joke to be a thing that brings the eight-year-old together with the 80-year-old," she states.

The Science Behind Communal Laughter

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

"Therefore when you are chuckling with people around the holiday dinner you are engaging in what's very likely a truly primordial mammalian social vocalisation," says a neuroscience expert.

Communal amusement, she says, helps forge and strengthen social connections between people.

Researchers have found that a lack of these social exchanges can seriously damage both psychological and bodily well-being.

"Those you talk to, and share laughter with, it leads to increased levels of 'happy chemical' uptake," the professor adds.

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

"You're not just laughing at a foolish pun with a Christmas cracker," she states. "You are actually performing a lot of the really important task of making, maintaining the social bonds you have with the people you care about."

Which Happens In the Brain?

But what is actually taking place inside the brain when we hear a joke?

An awful lot occurs in response to humour, it turns out.

Employing brain scanning technology, a type of brain scanner which indicates which areas of the brain are more active, researchers have been able to map the areas that receive more blood.

Testing entails scanning the brains of volunteer subjects and then exposing them to a database of humorous words, paired with either a neutral sound, or recorded laughter.

"In the scanner we got a really fascinating pattern of activation," notes the neuroscientist.

A joke activates not just the parts of the mind in charge of hearing and interpreting speech, but also brain areas associated with both planning and initiating movement and those involved in vision and recall.

Combine all of this as a whole, and people hearing a pun have a complex set of neural responses that support the amusement we experience.

The Infectious Power of Laughter

Scientists discovered that when a humorous phrase is paired with laughter there is a stronger reaction in the mind than the same phrase when accompanied by a non-emotional sound.

"This was in areas of the brain that you would use to contort your expression into a smile or a chuckle," the professor says.

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

Amusement, according to the expert, can be contagious.

So what does this imply for the laughter heard at a holiday table?

"People laugh harder when you know others," she notes, "and laughter increases more when you are fond of them or care for them."

When it comes to festive cracker puns, she explains, the positive effect is more likely to be caused not by the gag itself, but from the reaction to it.

"It's the laughter. The gag is the dreadful Christmas cracker pun, and it's just a pretext to chuckle as a group."

The Search for the Ideal Festive Pun

Is it possible to discover the ultimate joke?

Likely not, but that has not stopped researchers from trying to.

In 2001, a psychologist established a research project for the planet's funniest joke.

Over 40,000 gags submitted, with ratings provided by 350,000 people around the world, he has a better idea than many as to what works and what does not.

The perfect Christmas cracker pun must be brief, he says.

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

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

"The reason is that if no-one finds it funny – it's the gag's fault, not yours.

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

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