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

This joke is met by groans that resonate through a storage facility in London.

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

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

"The success is gauged by the joke by the volume of moans and the intensity of the groans around the table," the founder explains.

The secret to a great Christmas cracker joke is not the identical as a stand-up gag per se. It is entirely about the setting - in this case, the shared amusement of the Christmas meal with elders, children and potentially neighbours.

"You want the joke to be something that unites the eight-year-old together with the grandparent," she adds.

The Science Of Shared Amusement

Coming together to experience shared laughter is not only nothing new, experts say, it is probably to be older than humanity.

"Therefore when you are laughing with others around the holiday table you are dropping into what's almost certainly a really primordial mammalian social vocalisation," explains a professor.

Communal laughter, she explains, helps forge and strengthen social bonds between individuals.

Scientists have found that a lack of these interactions can significantly harm mental and physical health.

"Those you talk to, and share laughter with, it leads to increased amounts of endorphin uptake," the professor continues.

Endorphins are the body's "feel-good compounds" and are released both to reduce stress and pain and in reaction to pleasurable activities, such as chuckling with loved ones over a particularly awful Christmas cracker gag.

"You're not just chuckling at a foolish pun with a holiday cracker," she says. "You are actually doing a lot of the truly vital task of building, preserving the social bonds you have with the people you care about."

Which Happens Inside the Brain?

But what is actually taking place within the mind when we hear a joke?

A tremendous amount occurs in reaction to humour, it turns out.

Using functional magnetic resonance imaging (fMRI), a kind of brain scanner which shows which parts of the brain are more active, researchers have been able to chart the regions that get more blood flow.

The research involves scanning the minds of healthy participants and then subjecting them to a database of humorous phrases, paired with either a neutral sound, or pre-recorded laughter.

"During the study we observed a really interesting activation pattern of neural activity," notes the professor.

A joke activates not just the areas of the brain in charge of auditory processing and understanding speech, but also brain regions associated with both preparation and starting motion and those linked to sight and memory.

Combine these elements together, and people listening to a joke have a complex series of neural responses that underpin the laughter we experience.

The Infectious Nature of Chuckles

Researchers discovered that when a funny phrase is combined with chuckles there is a greater reaction in the mind than the same phrase when accompanied by a neutral sound.

"This activation occurred in parts of the mind that you would employ to contort your face into a grin or a laugh," the professor says.

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

Amusement, according to the professor, can be infectious.

So what does this imply for the laughter heard at a Christmas gathering?

"You laugh more when you are familiar with others," she says, "and laughter increases further when you are fond of them or love them."

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

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

The Search for the Perfect Festive Pun

Will we ever discover the ultimate joke?

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

In 2001, a professor established a research search for the planet's most humorous joke.

More than tens of thousands of jokes later, with ratings lodged by hundreds of thousands of participants globally, he has a better idea than many as to what succeeds and what does not.

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

"But they also need to be bad gags, puns that cause us to moan," he continues.

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

"This is because if no-one finds it funny – it's the gag's fault, not yours.

"What's interesting about the Christmas cracker jokes is that none of us find them funny.

"It creates a common moment around the table and I believe it's wonderful."