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

This joke is greeted with moans that resonate through a warehouse in the capital.

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

The firm's founder smiles, nearly apologetically at the joke. But the pun has made the cut and will appear in upcoming crackers.

"You measure the gag by the volume of moans and the loudness of the groans around the table," the founder explains.

The secret to a great Christmas cracker pun is not the same as a good joke per se. It is entirely about the setting - in this instance, the communal laughter of the Christmas meal with grandparents, kids and potentially friends.

"You want the gag to be something that brings the eight-year-old in harmony with the grandparent," she states.

The Science Of Shared Amusement

Coming together to experience communal amusement is not only nothing new, scientists argue, it is probably to be older than humanity.

"Therefore when you are chuckling with others at the holiday dinner you are engaging in what's almost certainly a truly ancient mammalian play vocalisation," says a professor.

Shared amusement, she explains, aids in make and maintain social connections between people.

Scientists have found that a lack of these social exchanges can seriously damage mental and physical health.

"The people you talk to, and share laughter with, it results in enhanced amounts of endorphin release," she adds.

Endorphins are the body's "happy chemicals" and are released both to alleviate tension and discomfort and in reaction to pleasurable experiences, such as chuckling with friends over a particularly terrible festive cracker joke.

"It's not simply laughing at a foolish joke with a holiday cracker," the expert says. "You are in fact doing a lot of the really vital task of building, preserving the connections you have with those you care about."

Which Happens In the Brain?

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

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

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

Testing entails imaging the minds of healthy subjects and then subjecting them to a database of funny words, accompanied by either a non-emotional sound, or pre-recorded laughter.

"In the scanner we observed a very fascinating activation pattern of neural activity," notes the neuroscientist.

A gag stimulates not just the areas of the brain in charge of auditory processing and understanding language, but also neural regions involved in both planning and initiating motion and those linked to sight and memory.

Put all of this together, and individuals hearing a pun have a complex series of neural responses that underpin the amusement we experience.

The Contagious Power of Chuckles

Researchers found that when a humorous word is combined with chuckles there is a greater response in the brain than the identical word when followed by a neutral sound.

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

It indicates we are not just reacting to funny words, they are responding to the laughter that follows them.

Amusement, according to the expert, can be contagious.

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

"You laugh more when you know others," she says, "and laughter increases more when you are fond of them or love them."

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

"The laughter is key. The joke is the terrible Christmas cracker pun, and it's just a pretext to chuckle as a group."

The Search for the Ideal Cracker Joke

Will we ever find the ultimate gag?

Probably not, but that has not prevented researchers from trying to.

Years ago, a psychologist established a scientific search for the world's funniest gag.

Over 40,000 gags later, with ratings lodged by hundreds of thousands of participants around the world, he has a clearer understanding than most as to what works and what fails.

The perfect festive cracker pun needs to be short, he says.

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

The increasingly "terrible" the joke, he says the better.

"The reason is that if no-one laughs – it's the gag's fault, not your own.

"The fascinating part about the holiday cracker jokes is that none of us find them funny.

"That's a shared moment at the gathering and I believe it's lovely."