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

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

This describes a joke-testing session with a company that makes supplies for social events. Its repertoire features festive crackers.

The firm's founder smiles, almost sheepishly at the joke. But the joke has made the cut and will feature in upcoming crackers.

"The success is gauged by the gag by the number of moans and the loudness of the groans around the table," she says.

The secret to a great Christmas cracker joke is not the same as a stand-up gag per se. It is all about the setting - in this case, the shared laughter of the Christmas dinner table with grandparents, children and potentially friends.

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

The Neuroscience Of Shared Amusement

Gathering to experience communal amusement is not only ancient, experts say, it is likely to be older than humanity.

"Therefore when you are laughing with others at the Christmas dinner you are engaging in what's almost certainly a truly primordial mammal play vocalisation," says a neuroscience expert.

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

Scientists have found that a lack of such interactions can seriously harm mental and physical well-being.

"The people you talk to, and laugh with, it leads to increased levels of endorphin release," the professor adds.

Endorphins are the body'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 particularly awful Christmas cracker gag.

"You're not just chuckling at a foolish joke with a holiday cracker," the expert states. "You are in fact performing a lot of the really vital work of making, maintaining the connections you have with those you care about."

Which Happens In the Mind?

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

A tremendous amount happens in response to humour, it transpires.

Using functional magnetic resonance imaging (fMRI), a kind of brain scanner which shows which areas of the mind are working harder, scientists have been able to chart the areas that receive more blood.

The research involves scanning the brains of volunteer participants and then exposing them to a collection of funny phrases, accompanied by either a neutral sound, or pre-recorded chuckles.

"During the study we observed a really fascinating pattern of activation," says the professor.

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

Combine all of this as a whole, and individuals hearing a joke have a sophisticated series of brain reactions that underpin the amusement we experience.

The Contagious Power of Chuckles

Scientists found that when a funny phrase is combined with chuckles there is a stronger reaction in the brain than the identical word when accompanied by a neutral sound.

"This was in areas of the brain that you would use to move your face into a grin or a chuckle," the professor says.

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

Laughter, according to the professor, can be contagious.

So what does this mean for the chuckles found at a Christmas gathering?

"People laugh more when you know others," she says, "and you laugh more when you like them or love them."

When it comes to festive cracker jokes, she explains, the feel-good effect is more probable to be caused not by the gag itself, but from the response to it.

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

The Search for the Ideal Festive Pun

Will we ever find the perfect joke?

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

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

More than tens of thousands of jokes later, with scores provided by 350,000 people globally, he has a better understanding than many as to what works and what fails.

The ideal festive cracker joke needs to be short, he says.

"They must also need to be bad gags, puns that cause us to moan," he adds.

The more "terrible" the gag, he says the more effective.

"This is because if no-one laughs – it's the joke's shortcoming, not your own.

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

"It creates a shared experience around the table and I think it's lovely."