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

This quip is greeted with moans that resonate through a storage facility in the capital.

We're at a humor-evaluation meeting with a firm that makes products for gatherings. Its repertoire includes Christmas crackers.

The firm's owner smiles, almost sheepishly at the gag. But the pun has been selected and will feature in future crackers.

"The success is gauged by the joke by the volume of moans and the loudness of the groans at the table," she says.

The secret to a good Christmas cracker joke is not the identical as a stand-up joke per se. It is all about the context - in this case, the communal laughter of the holiday meal with elders, children and possibly neighbours.

"You want the gag to be a thing that unites the eight-year-old together with the grandparent," she adds.

The Neuroscience Of Shared Laughter

Coming together to experience communal laughter is not only nothing new, experts say, it is likely to be pre-human.

"Therefore when you are chuckling with people at the Christmas dinner you are engaging in what's almost certainly a truly ancient mammalian play sound," says a neuroscience expert.

Shared laughter, she explains, helps make and maintain social bonds between people.

Scientists have found that a lack of these social exchanges can seriously damage mental and physical health.

"The people you converse with, and laugh with, it leads to enhanced levels of endorphin uptake," the professor continues.

These natural chemicals are the brain's "happy chemicals" and are released both to reduce tension and discomfort and in response to pleasurable experiences, such as laughing with loved ones over a truly awful Christmas cracker joke.

"You're not just laughing at a silly pun with a Christmas cracker," the expert states. "You are in fact performing a lot of the truly important task of making, maintaining the social bonds you have with the people you care about."

Which Occurs Inside the Mind?

But what is truly happening within the brain when we hear a joke?

An awful lot occurs in response to humour, it transpires.

Employing brain scanning technology, a kind of brain scanner which shows which areas of the brain are more active, researchers have been able to map the areas that receive more blood.

The research involves scanning the minds of healthy subjects and then subjecting them to a collection of humorous words, paired with either a non-emotional sound, or pre-recorded laughter.

"In the scanner we got a very fascinating pattern of neural activity," says the neuroscientist.

A joke activates not just the areas of the mind responsible for hearing and interpreting speech, but also neural regions involved in both preparation and starting movement and those involved in sight and memory.

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

The Infectious Power of Chuckles

Scientists found that when a funny phrase is combined with chuckles there is a greater reaction in the brain than the identical word when followed by a neutral sound.

"This activation occurred in areas of the mind that you would use to move your face into a grin or a chuckle," she explains.

It means people are not just reacting to funny words, they are responding to the amusement that accompanies them.

Amusement, according to the professor, can be contagious.

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

"People laugh harder when you are familiar with others," she says, "and laughter increases further when you like them or care for them."

When it comes to festive cracker jokes, she explains, the positive factor is more likely to be triggered not by the joke in 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 laugh together."

The Search for the Ideal Cracker Joke

Will we ever discover the perfect gag?

Likely not, but that has not stopped experts from trying to.

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

Over tens of thousands of gags submitted, with scores lodged by 350,000 people globally, he has a clearer idea than most as to what succeeds and what does not.

The ideal festive cracker pun needs to be brief, he explains.

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

The more "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 puns is that none of us find them funny.

"That's a common moment at the gathering and I think it's wonderful."