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"What was the price did Father Christmas's sled cost? Nothing, it was on the house."

This one-liner is met by groans that resonate through a warehouse in the capital.

This describes a humor-evaluation session with a firm that makes products for gatherings. Its catalogue features Christmas crackers.

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

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

The key to a good Christmas cracker joke is not the same as a good joke in itself. It is entirely about the context - in this case, the communal amusement of the holiday dinner table with grandparents, kids and potentially friends.

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

The Science Of Shared Laughter

Coming together to experience shared laughter is not only ancient, scientists argue, it is probably to be pre-human.

"Therefore when you are chuckling with others around the Christmas dinner you are engaging in what's very likely a truly primordial mammalian play vocalisation," explains a professor.

Shared amusement, she says, helps forge and strengthen social bonds between individuals.

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

"The people you talk to, and share laughter with, it leads to enhanced amounts of endorphin release," the professor continues.

Endorphins are the brain's "happy chemicals" and are released both to alleviate stress and pain and in response to pleasurable experiences, such as chuckling with friends over a truly terrible Christmas cracker gag.

"You're not just chuckling at a silly pun with a holiday cracker," she states. "You are actually doing a lot of the truly vital work of building, preserving the connections you have with those you care about."

Which Happens Inside the Mind?

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

An awful lot happens in response to comedy, it transpires.

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

Testing entails imaging the minds of volunteer subjects and then subjecting them to a collection of humorous words, paired with either a non-emotional sound, or recorded chuckles.

"During the study we observed a really interesting pattern of neural activity," says the neuroscientist.

A joke activates not just the parts of the brain in charge of hearing and interpreting speech, but also brain regions involved in both preparation and starting motion and those linked to sight and recall.

Put these elements as a whole, and people hearing a pun have a sophisticated set of brain reactions that support the amusement we hear.

The Infectious Nature of Chuckles

Scientists discovered that when a humorous word is paired with laughter there is a greater response in the mind than the same word when accompanied by a non-emotional sound.

"This activation occurred in parts of the brain that you would use to move your face into a grin or a laugh," she says.

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

Laughter, says the professor, can be contagious.

So what does this mean for the chuckles heard around a Christmas table?

"You laugh harder when you know others," she says, "and you laugh further when you are fond of them or care for them."

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

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

The Quest for the Perfect Cracker Joke

Is it possible to find the perfect joke?

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

Years ago, a professor set up a scientific search for the planet's most humorous gag.

Over tens of thousands of gags later, with scores lodged by hundreds of thousands of people globally, he has a better understanding than most as to what works and what fails.

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

"But they also be poor jokes, jokes that make us groan," he adds.

The more "awful" the joke, he says the more effective.

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

"The fascinating part about the Christmas cracker puns is that not one person find them funny.

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