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"How much did Father Christmas's sleigh cost? Nothing, it was on the house."

This quip is met by groans that echo through a storage facility in the capital.

We're at a humor-evaluation session with a company that produces products for gatherings. Its repertoire features festive crackers.

The company's founder grins, almost apologetically at the joke. But the joke has been selected and will appear in future crackers.

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

The secret to a great Christmas cracker pun is not the same as a stand-up joke per se. It is entirely about the setting - in this case, the shared amusement of the holiday meal with elders, children and possibly friends.

"The goal is for the joke to be a thing that unites the eight-year-old in harmony with the 80-year-old," she adds.

The Neuroscience Behind Shared Laughter

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

"Therefore when you are laughing with people at the holiday dinner you are dropping into what's very likely a truly primordial mammalian social vocalisation," says a professor.

Shared amusement, she says, helps make and maintain social bonds between individuals.

Scientists have discovered that a lack of such social exchanges can seriously damage both psychological and bodily health.

"Those you converse with, and laugh with, it results in enhanced amounts of endorphin release," the professor continues.

Endorphins are the body's "happy chemicals" and are produced both to alleviate stress and pain and in response to pleasurable experiences, such as laughing with loved ones over a particularly awful festive cracker joke.

"It's not simply chuckling at a foolish pun with a holiday cracker," she says. "You are in fact doing a lot of the really important work of making, maintaining the social bonds you have with those you love."

What Happens Inside the Brain?

But what is actually happening inside the mind when we listen to a gag?

A tremendous amount occurs in reaction to humour, it transpires.

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

The research involves imaging the minds of volunteer subjects and then subjecting them to a collection of humorous words, accompanied by either a neutral sound, or pre-recorded chuckles.

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

A joke activates not just the parts of the brain in charge of hearing and understanding speech, but also brain areas associated with both planning and initiating motion and those involved in sight and memory.

Combine all of this as a whole, and individuals hearing a joke have a complex set of brain responses that underpin the amusement we experience.

The Contagious Power of Laughter

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

"This was in parts of the brain that you would employ to move your expression into a smile or a laugh," the professor says.

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

Amusement, says the expert, can be contagious.

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

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

When it comes to Christmas cracker puns, she explains, the feel-good factor is more probable to be triggered not by the gag in itself, but from the reaction to it.

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

The Quest for the Ideal Festive Pun

Is it possible to find the ultimate joke?

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

Years ago, a professor set up a scientific project for the planet's funniest joke.

More than 40,000 jokes later, with scores provided by hundreds of thousands of participants globally, he has a clearer idea than most as to what succeeds and what fails.

The perfect festive cracker joke needs to be brief, he explains.

"But they also be bad gags, jokes that cause us to moan," he continues.

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

"The reason is that if no-one laughs – it's the joke's fault, not yours.

"What's interesting about the Christmas cracker puns is that none of us find them funny.

"That's a shared moment around the table and I think it's lovely."