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

This joke is greeted with groans that resonate through a storage facility in London.

We're at a joke-testing session with a company that produces products for social events. Its repertoire includes Christmas crackers.

The company's owner grins, 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 volume of moans and the intensity of the groans at the table," the founder says.

The key to a good Christmas cracker joke is not the identical as a good joke per se. It is all about the setting - in this case, the shared laughter of the Christmas meal with grandparents, children and possibly neighbours.

"You want the joke to be a thing that brings the eight-year-old in harmony with the 80-year-old," she states.

The Neuroscience Of Communal Laughter

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

"Therefore when you are laughing with others at the holiday dinner you are engaging in what's almost certainly a truly ancient mammal social sound," explains a neuroscience expert.

Shared laughter, she says, aids in make and maintain social connections between individuals.

Scientists have found that a absence of these social exchanges can significantly harm mental and physical well-being.

"Those you converse with, and laugh with, it results in increased levels 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.

"You're not just chuckling at a silly pun with a holiday cracker," the expert says. "You are actually doing a lot of the really vital work of making, maintaining the social bonds you have with those you care about."

What Occurs In the Brain?

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

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

Using functional magnetic resonance imaging (fMRI), a type of brain scanner which indicates which parts of the mind are more active, researchers have been able to map the regions that receive more blood flow.

Testing entails scanning the minds of volunteer subjects and then subjecting them to a collection of funny words, paired with either a non-emotional sound, or pre-recorded chuckles.

"During the study we got a very fascinating pattern of neural activity," notes the professor.

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 starting movement and those linked to vision and recall.

Put all of this together, and people listening to a joke have a complex set of brain responses that underpin the amusement we experience.

The Infectious Power of Chuckles

Researchers found that when a funny word is paired with laughter there is a stronger reaction in the brain than the same word when accompanied by a non-emotional sound.

"This was in parts of the brain that you would use to contort your expression into a smile or a laugh," she 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 infectious.

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

"You laugh harder when you are familiar with others," she notes, "and laughter increases further when you are fond of them or care for them."

When it comes to Christmas cracker jokes, she says, the positive effect is more likely to be triggered not by the gag in 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 discover the ultimate gag?

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

Years ago, a professor set up a scientific project for the world's funniest gag.

Over tens of thousands of jokes later, with ratings lodged by hundreds of thousands of people around the world, he has a better idea than most as to what works and what fails.

The perfect Christmas cracker joke needs to be brief, he explains.

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

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

"This is because if nobody finds it funny – it's the joke's fault, not yours.

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

"That's a shared experience at the gathering and I believe it's wonderful."