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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 echo through a warehouse in London.

We're at a joke-testing session with a company that makes supplies for social events. Its catalogue features festive crackers.

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

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

The secret to a good Christmas cracker joke is not the same as a stand-up joke in itself. It is all about the setting - in this case, the shared amusement of the Christmas meal with grandparents, kids and potentially friends.

"The goal is for the joke to be a thing that brings the child together with the grandparent," she states.

The Neuroscience Of Shared Laughter

Coming together to enjoy communal laughter is not only ancient, experts say, it is likely to be pre-human.

"So when you are chuckling with others at the holiday table you are engaging in what's almost certainly a really primordial mammalian social sound," says a professor.

Shared laughter, she says, aids in forge and strengthen social connections between individuals.

Researchers have discovered that a absence of such interactions can significantly harm mental and physical health.

"Those you converse with, and laugh with, it leads to increased amounts of endorphin uptake," the professor continues.

Endorphins are the brain's "happy chemicals" and are released both to reduce stress and pain and in response to enjoyable experiences, such as laughing with loved ones over a truly awful festive cracker joke.

"You're not just chuckling at a silly joke with a holiday cracker," she states. "You are in fact performing a lot of the truly important work of making, maintaining the connections you have with the people you love."

Which Occurs Inside the Brain?

But what is actually taking place within the brain when we hear a gag?

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

Using functional magnetic resonance imaging (fMRI), a type of brain scanner which shows which parts of the brain are working harder, researchers have been able to chart the areas that get more blood.

The research entails scanning the brains of healthy subjects and then exposing them to a collection of humorous words, accompanied by either a neutral sound, or pre-recorded laughter.

"In the scanner we observed a very interesting activation pattern of neural activity," says the professor.

A joke stimulates not just the areas of the brain responsible for hearing and understanding language, but also brain regions involved in both preparation and initiating movement and those linked to vision and memory.

Put all of this as a whole, and individuals listening to a joke have a complex set of brain reactions that support the laughter we hear.

The Contagious Power of Chuckles

Researchers discovered that when a humorous word is paired with chuckles there is a greater reaction in the mind than the identical word when followed by a non-emotional sound.

"This activation occurred in parts of the mind that you would employ to move your expression into a grin or a chuckle," the professor says.

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

Amusement, says the expert, can be infectious.

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

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

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

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

The Quest for the Ideal Cracker Joke

Is it possible to find the ultimate joke?

Likely not, but that has not prevented researchers from attempting to.

In 2001, a professor set up a research search for the planet's funniest gag.

More than tens of thousands of jokes later, with scores lodged by hundreds of thousands of people globally, he has a better idea than most as to what succeeds and what fails.

The ideal Christmas cracker joke must be brief, he explains.

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

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

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

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

"That's a common experience at the gathering and I think it's lovely."