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

This one-liner is met by groans that echo through a warehouse in London.

This describes a joke-testing session with a firm that makes products for gatherings. Its repertoire includes Christmas crackers.

The company's owner grins, nearly apologetically at the joke. But the joke has been selected and will appear 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 explains.

The key to a great Christmas cracker joke is not the same as a good gag per se. It is entirely about the context - in this instance, the communal laughter of the Christmas dinner table with grandparents, kids and potentially neighbours.

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

The Neuroscience Behind Communal Laughter

Gathering to enjoy communal laughter is not only ancient, scientists argue, it is likely to be older than humanity.

"Therefore when you are laughing with others around the Christmas dinner you are engaging in what's very likely a truly ancient mammal play sound," says a professor.

Communal laughter, she says, helps make and maintain social connections between people.

Researchers have discovered that a lack of these social exchanges can significantly damage mental and physical well-being.

"Those you talk to, and share laughter with, it results in enhanced levels of 'happy chemical' uptake," the professor adds.

These natural chemicals are the body's "feel-good compounds" and are produced both to alleviate stress and pain and in reaction to pleasurable activities, such as laughing with loved ones over a particularly awful Christmas cracker joke.

"You're not just chuckling at a silly pun with a holiday cracker," the expert states. "You are in fact performing a lot of the truly vital task of building, preserving the connections you have with those you love."

What Occurs In the Brain?

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

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

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

Testing entails imaging the brains of volunteer subjects and then exposing them to a collection of funny phrases, accompanied by either a non-emotional sound, or pre-recorded chuckles.

"During the study we got a really interesting activation pattern of activation," notes the professor.

A joke activates not just the parts of the brain in charge of auditory processing and understanding language, but also brain areas associated with both planning and starting motion and those involved in sight and recall.

Put these elements as a whole, and people listening to a pun have a sophisticated set of neural responses that support the amusement we experience.

The Contagious Nature of Chuckles

Scientists found that when a humorous word is combined with laughter there is a stronger reaction in the brain than the identical phrase when followed by a non-emotional sound.

"This activation occurred in areas of the brain that you would employ to contort your expression into a smile or a chuckle," the professor says.

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

Laughter, says the professor, can be infectious.

So what does this imply for the laughter found around a holiday gathering?

"People laugh harder when you are familiar with people," she notes, "and you laugh more when you like them or care for them."

When it comes to festive cracker jokes, she explains, the feel-good factor is more likely to be caused not by the gag itself, but from the response to it.

"The laughter is key. The gag is the dreadful holiday cracker pun, and it's just a reason to laugh together."

The Quest for the Ideal Cracker Joke

Will we ever find the ultimate gag?

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

In 2001, a psychologist established a research search for the planet's most humorous gag.

More than 40,000 gags later, with scores lodged by 350,000 participants globally, he has a better understanding than most as to what works and what does not.

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

"But they also be poor gags, jokes that cause us to moan," he continues.

The increasingly "terrible" the gag, he states the better.

"The reason is that if no-one finds it funny – it's the joke's fault, not your own.

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

"That's a common moment around the table and I think it's lovely."