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

This quip is met by moans that echo through a warehouse in the capital.

We're at a joke-testing meeting with a company that produces supplies for social events. Its repertoire includes festive crackers.

The firm's owner grins, nearly sheepishly at the gag. But the joke has been selected and will feature in upcoming crackers.

"The success is gauged by the joke by the number of groans and the intensity of the groans at the table," she says.

The key to a great Christmas cracker joke is not the identical as a good gag per se. It is entirely about the context - in this case, the shared laughter of the holiday meal with elders, kids and possibly friends.

"You want the joke to be something that unites the eight-year-old together with the 80-year-old," she states.

The Neuroscience Behind Shared Amusement

Coming together to enjoy shared amusement is not only nothing new, experts say, it is probably to be older than humanity.

"So when you are chuckling with others around the Christmas table you are dropping into what's very likely a really ancient mammal play vocalisation," explains a neuroscience expert.

Communal laughter, she says, helps make and maintain social connections between individuals.

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

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

Endorphins are the brain's "feel-good compounds" and are released both to reduce stress and pain and in response to pleasurable activities, such as laughing with friends over a particularly awful Christmas cracker gag.

"It's not simply laughing at a silly joke with a holiday cracker," the expert says. "You are actually performing a lot of the really vital work of making, maintaining the social bonds you have with those you care about."

What Occurs Inside the Brain?

But what is actually taking place within the brain when we listen to a joke?

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

Employing functional magnetic resonance imaging (fMRI), a kind of brain scanner which indicates which areas of the mind are more active, scientists have been able to map the regions that receive more blood.

The research involves scanning the brains of volunteer subjects and then subjecting them to a database of humorous phrases, accompanied by either a neutral sound, or pre-recorded laughter.

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

A gag activates not just the areas of the brain in charge of hearing and interpreting speech, but also neural regions associated with both preparation and starting movement and those linked to sight and memory.

Put these elements together, and people hearing a joke have a complex set of brain responses that underpin the amusement we experience.

The Contagious Nature of Laughter

Researchers found that when a funny word is paired with chuckles there is a greater response in the brain than the same word when followed by a neutral sound.

"This was in parts of the mind that you would employ to contort your expression into a grin or a chuckle," she explains.

It means people are not just reacting to humorous jokes, they are responding 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 table?

"You laugh more when you are familiar with others," she notes, "and laughter increases more when you like them or love them."

When it comes to Christmas cracker jokes, she explains, the positive effect is more likely to be triggered not by the gag itself, but from the reaction to it.

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

The Search for the Ideal Cracker Joke

Will we ever discover the ultimate joke?

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

Years ago, a psychologist established a research project for the world's funniest gag.

More than tens of thousands of jokes later, with ratings lodged by hundreds of thousands of participants around the world, he has a clearer understanding than many as to what works and what fails.

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

"They must also be bad jokes, jokes that make us groan," he adds.

The more "awful" the joke, he states the better.

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

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

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