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

This one-liner is greeted with groans that resonate through a storage facility in London.

We're at a humor-evaluation meeting with a firm that produces supplies for social events. Its catalogue includes festive crackers.

The firm's owner grins, almost apologetically at the gag. But the joke has made the cut and will feature in upcoming crackers.

"You measure the gag by the number of moans and the intensity of the groans around the table," the founder says.

The key to a good holiday cracker joke is not the same as a stand-up gag in itself. It is all about the context - in this instance, the communal amusement of the Christmas meal with grandparents, children and potentially 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 experience shared amusement is not only nothing new, scientists say, it is probably to be older than humanity.

"Therefore when you are laughing with people at the Christmas dinner you are engaging in what's very likely a really ancient mammalian social vocalisation," says a neuroscience expert.

Communal amusement, she explains, helps forge and strengthen social connections between people.

Researchers have discovered that a lack of such interactions can significantly harm both psychological and bodily health.

"Those you converse with, and share laughter with, it leads to increased amounts of 'happy chemical' uptake," the professor adds.

Endorphins are the brain's "feel-good compounds" and are released both to reduce stress and pain and in response to enjoyable activities, such as laughing with loved ones over a truly awful Christmas cracker joke.

"It's not simply laughing at a foolish joke with a holiday cracker," she says. "You are actually performing a lot of the really vital task of making, maintaining the connections you have with those you love."

Which Occurs Inside the Brain?

But what is truly taking place within the mind when we hear a gag?

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

Employing brain scanning technology, a type of brain scanner which indicates which parts of the brain are more active, researchers have been able to chart the regions that receive more blood.

Testing entails imaging the minds of healthy subjects and then exposing them to a collection of humorous phrases, accompanied by either a neutral sound, or pre-recorded chuckles.

"During the study we observed a very interesting pattern of activation," notes the neuroscientist.

A joke stimulates not just the parts of the brain responsible for hearing and understanding speech, but also brain regions associated with both preparation and initiating motion and those involved in sight and recall.

Put these elements as a whole, and individuals hearing a pun have a sophisticated series of neural responses that underpin the laughter we experience.

The Infectious Power of Laughter

Scientists found that when a funny word 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 areas of the mind that you would employ to contort your face into a grin or a chuckle," the professor says.

It means we are not just responding to humorous jokes, they are responding to the laughter that follows them.

Laughter, according to the expert, can be contagious.

So what does this imply for the chuckles found around a Christmas gathering?

"People laugh more when you know people," she notes, "and laughter increases more when you like them or care for them."

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

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

The Quest for the Perfect Cracker Joke

Will we ever find the ultimate gag?

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

Years ago, a professor established a scientific search for the planet's funniest joke.

Over tens of thousands of jokes later, with ratings lodged by 350,000 people around the world, he has a clearer understanding than many as to what succeeds and what fails.

The perfect festive cracker pun must be brief, he explains.

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

The increasingly "terrible" the gag, he states the more effective.

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

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

"That's a shared experience around the table and I believe it's wonderful."