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

This one-liner is greeted with moans that resonate through a storage facility in London.

This describes a joke-testing session with a company that produces supplies for social events. Its repertoire includes Christmas crackers.

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

"You measure the gag by the volume of groans and the intensity of the groans around the table," she explains.

The key to a great Christmas cracker pun is not the identical as a good gag in itself. It is entirely about the setting - in this instance, the communal amusement of the holiday dinner table with grandparents, children and potentially friends.

"The goal is for the joke to be a thing that unites the child together with the 80-year-old," she adds.

The Science Behind Shared Laughter

Gathering to experience communal amusement is not only ancient, experts argue, it is probably to be older than humanity.

"Therefore when you are chuckling with people at the Christmas dinner you are dropping into what's almost certainly a really primordial mammalian play sound," says a neuroscience expert.

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

Researchers have discovered that a absence of such social exchanges can significantly harm mental and physical well-being.

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

Endorphins are the brain's "feel-good compounds" and are released both to reduce stress and pain and in response to enjoyable experiences, such as chuckling with friends over a truly awful Christmas cracker gag.

"You're not just laughing at a foolish pun with a holiday cracker," she states. "You are in fact performing a lot of the really important work of building, preserving the connections you have with those you care about."

What Occurs Inside the Mind?

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

An awful lot 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 mind are more active, scientists have been able to map the regions that receive more blood.

The research entails scanning the brains of healthy participants and then exposing them to a collection of funny phrases, accompanied by either a neutral sound, or recorded chuckles.

"During the study we observed a very interesting pattern of activation," says the professor.

A joke stimulates not just the areas of the brain in charge of auditory processing and understanding language, but also neural regions associated with both preparation and starting movement and those linked to vision and memory.

Put these elements as a whole, and individuals hearing a joke have a sophisticated set of neural responses that support the amusement we experience.

The Infectious Power of Chuckles

Scientists found that when a funny word is paired with laughter there is a greater reaction in the mind than the same phrase when accompanied by a non-emotional sound.

"This activation occurred in parts of the brain that you would use to contort your face into a smile or a laugh," the professor explains.

It means we are not just responding to humorous jokes, they are responding to the amusement that follows them.

Laughter, says the professor, can be contagious.

So what does this mean for the laughter found around a Christmas gathering?

"People laugh harder when you know people," she notes, "and you laugh further when you are fond of them or care for them."

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

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

The Quest for the Perfect Cracker Joke

Is it possible to discover the perfect joke?

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

In 2001, a psychologist set up a scientific search for the planet's funniest joke.

More than tens of thousands of jokes submitted, with scores provided by 350,000 people globally, he has a better understanding than many as to what succeeds and what does not.

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

"They must also need to be poor gags, jokes that cause us to moan," he adds.

The more "awful" the joke, he says the more effective.

"This is because if nobody finds it funny – it's the gag's fault, not your own.

"The fascinating part about the holiday cracker jokes is that none of us find them funny.

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