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"How much did Santa's sleigh cost? Zero, it was on the house."

This one-liner is greeted with moans that echo through a storage facility in London.

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

The company's owner smiles, nearly apologetically 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 moans and the loudness of the groans at the table," she explains.

The secret to a great holiday cracker pun is not the identical as a good gag per se. It is all about the setting - in this case, the shared laughter of the holiday dinner table with elders, kids and possibly neighbours.

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

The Science Of Shared Laughter

Gathering to enjoy shared amusement is not only ancient, experts say, it is likely to be pre-human.

"Therefore when you are chuckling with people at the holiday dinner you are dropping into what's very likely a really ancient mammalian play sound," says a neuroscience expert.

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

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

"Those you talk to, and share laughter with, it results in enhanced levels of endorphin uptake," she adds.

Endorphins are the body's "feel-good compounds" and are released both to alleviate tension and discomfort and in reaction to enjoyable experiences, such as chuckling with loved ones over a particularly awful festive cracker gag.

"It's not simply chuckling at a foolish joke with a Christmas cracker," the expert states. "You are actually doing a lot of the truly vital task of making, maintaining the connections you have with those you love."

What Happens Inside the Mind?

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

An awful lot happens in response to humour, it turns out.

Using functional magnetic resonance imaging (fMRI), a kind of neural imager which indicates which parts of the brain are more active, scientists have been able to map the areas that get more blood.

The research entails scanning the minds of volunteer subjects and then subjecting them to a database of humorous words, paired with either a neutral sound, or recorded laughter.

"In the scanner we got a really fascinating pattern of neural activity," says the professor.

A joke activates not just the parts of the mind in charge of auditory processing and understanding language, but also brain regions involved in both preparation and starting movement and those involved in vision and recall.

Combine all of this together, and people listening to a joke have a sophisticated set of neural responses that underpin the amusement we hear.

The Infectious Power of Chuckles

Scientists found that when a funny phrase is paired with laughter 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 use to move your expression into a grin or a chuckle," she explains.

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

Amusement, according to the professor, can be contagious.

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

"You laugh more when you know people," she notes, "and you laugh more when you are fond of them or love them."

When it comes to festive cracker puns, she says, the feel-good factor is more probable to be caused not by the joke itself, but from the response to it.

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

The Quest for the Perfect Festive Pun

Will we ever find the perfect joke?

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

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

More than 40,000 gags later, with scores lodged by hundreds of thousands of participants around the world, he has a better understanding than many as to what succeeds and what does not.

The ideal festive cracker joke must be short, he says.

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

The increasingly "awful" the gag, he says the more effective.

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

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

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