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

This one-liner is met by moans that echo through a storage facility in London.

This describes a joke-testing meeting with a firm that produces supplies for gatherings. Its repertoire includes festive crackers.

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

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

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

"You want the joke to be something that unites the child in harmony with the 80-year-old," she states.

The Neuroscience Behind Shared Amusement

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

"So when you are laughing with people at the Christmas dinner you are engaging in what's very likely a truly ancient mammalian social vocalisation," explains a neuroscience expert.

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

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

"The people you talk to, and share laughter with, it leads to enhanced amounts of 'happy chemical' uptake," she continues.

Endorphins are the brain's "happy chemicals" and are produced both to reduce tension and discomfort and in reaction to enjoyable experiences, such as laughing with friends over a particularly awful Christmas cracker joke.

"It's not simply laughing at a foolish pun with a holiday cracker," the expert says. "You are in fact doing a lot of the really important work of building, preserving the social bonds you have with the people you love."

What Occurs In the Mind?

But what is truly happening inside the brain when we listen to a gag?

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

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

The research entails imaging the brains of healthy subjects and then exposing them to a collection of humorous phrases, accompanied by either a non-emotional sound, or pre-recorded laughter.

"During the study we observed a really fascinating activation pattern of neural activity," notes the neuroscientist.

A joke activates not just the parts of the mind responsible for auditory processing and understanding speech, but also neural regions involved in both planning and starting motion and those involved in vision and memory.

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

The Infectious Nature of Laughter

Scientists found that when a funny phrase is paired with chuckles there is a greater reaction in the mind than the same word when accompanied by a neutral sound.

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

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

Amusement, according to the professor, can be infectious.

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

"You laugh more when you know people," she says, "and you laugh further when you like them or love them."

When it comes to Christmas cracker jokes, she says, the feel-good factor is more probable to be caused not by the gag in itself, but from the response to it.

"It's the laughter. The joke is the dreadful holiday cracker pun, and it's just a reason to laugh together."

The Quest for the Ideal Cracker Joke

Is it possible to discover the ultimate gag?

Probably not, but that has not stopped experts from trying to.

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

More than tens of thousands of jokes later, with scores lodged by hundreds of thousands of people globally, he has a better understanding than most as to what succeeds and what does not.

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

"But they also need to be poor gags, jokes that make us moan," he adds.

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

"This is because if nobody finds it funny – it's the joke's fault, not yours.

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

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