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

This one-liner is met by moans that resonate through a storage facility in London.

We're at a joke-testing meeting with a company that makes products for gatherings. Its catalogue includes festive crackers.

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

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

The key to a great Christmas cracker joke is not the same as a stand-up gag in itself. It is entirely about the setting - in this case, the communal laughter of the holiday dinner table with elders, kids and possibly neighbours.

"The goal is for the gag to be something that unites the child in harmony with the 80-year-old," she states.

The Neuroscience Of Shared Laughter

Coming together to enjoy shared laughter is not only ancient, scientists argue, it is probably to be older than humanity.

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

Communal amusement, she explains, helps make and maintain social connections between people.

Researchers have discovered that a lack of these social exchanges can seriously harm both psychological and bodily well-being.

"Those you talk to, and share laughter with, it results in increased levels of endorphin uptake," the professor adds.

These natural chemicals are the body's "happy chemicals" and are produced both to reduce tension and discomfort and in response to pleasurable activities, such as chuckling with friends over a truly terrible festive cracker gag.

"It's not simply laughing at a foolish joke with a Christmas cracker," she states. "You are actually performing a lot of the truly vital task of building, preserving the connections you have with the people you care about."

Which Occurs In the Brain?

But what is actually taking place within the mind when we listen to a gag?

A tremendous amount happens in response to humour, it turns out.

Using brain scanning technology, a type of brain scanner which shows which areas of the brain are working harder, scientists have been able to map the regions that get more blood.

The research involves scanning the brains of volunteer subjects and then exposing them to a collection of funny phrases, paired with either a non-emotional sound, or recorded laughter.

"During the study we observed a really fascinating activation pattern of activation," notes the professor.

A joke stimulates not just the areas of the brain responsible for hearing and understanding language, but also brain regions involved in both preparation and starting motion and those involved in sight and memory.

Put these elements together, and individuals hearing a pun have a sophisticated set of brain responses that support the amusement we hear.

The Contagious Nature of Chuckles

Researchers found that when a humorous phrase is combined with chuckles there is a greater reaction in the mind than the identical word when accompanied by a non-emotional sound.

"This was in areas of the brain that you would employ to move your face into a grin or a laugh," the professor explains.

It means we are not just reacting to humorous words, they are responding to the laughter that accompanies them.

Laughter, says the professor, can be contagious.

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

"People laugh harder when you know people," she notes, "and you laugh more when you like them or love them."

When it comes to Christmas cracker puns, 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 gag is the dreadful holiday cracker joke, and it's just a pretext to chuckle as a group."

The Quest for the Perfect Festive Pun

Is it possible to discover the ultimate joke?

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

Years ago, a professor set up a scientific project for the planet's funniest joke.

More than 40,000 gags later, with ratings provided by hundreds of thousands of participants globally, he has a clearer understanding than most as to what works and what does not.

The perfect festive cracker pun must be short, he says.

"They must also be poor gags, puns that cause us to moan," he continues.

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

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

"The fascinating part about the Christmas cracker jokes is that not one person find them funny.

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