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

This one-liner is greeted with moans that echo through a warehouse in London.

We're at a joke-testing session with a company that produces products for social events. Its repertoire features festive crackers.

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

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

The key to a great holiday cracker pun is not the identical as a stand-up joke in itself. It is entirely about the setting - in this instance, the communal amusement of the holiday meal with elders, kids and possibly friends.

"You want the gag to be something that unites the child in harmony with the 80-year-old," she states.

The Neuroscience Behind Communal Amusement

Gathering to enjoy shared amusement is not only ancient, scientists say, it is probably to be pre-human.

"So when you are chuckling with others around the holiday dinner you are dropping into what's very likely a really primordial mammal play sound," says a neuroscience expert.

Communal laughter, she says, helps forge and strengthen social connections between people.

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

"Those you talk to, and laugh with, it leads to increased levels of 'happy chemical' release," she continues.

Endorphins are the brain's "feel-good compounds" and are produced both to reduce tension and discomfort and in response to enjoyable activities, such as chuckling with friends over a particularly awful festive cracker joke.

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

Which Happens In the Mind?

But what is actually happening 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 neural imager which indicates which parts of the brain are working harder, researchers have been able to chart the regions that get more blood flow.

The research involves imaging the brains of volunteer participants and then subjecting them to a database of funny phrases, accompanied by either a neutral sound, or recorded laughter.

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

A gag stimulates not just the parts of the mind in charge of hearing and understanding speech, but also neural regions associated with both planning and starting movement and those involved in sight and memory.

Put all of this as a whole, and individuals hearing a pun have a sophisticated set of brain reactions that support the amusement we hear.

The Contagious Power of Chuckles

Scientists found that when a humorous phrase is paired with laughter there is a greater reaction in the brain than the same phrase when followed by a non-emotional sound.

"This was in parts of the mind that you would employ to move your expression into a smile or a laugh," she explains.

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

Amusement, according to the professor, can be infectious.

So what does this mean for the chuckles found at a holiday table?

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

When it comes to festive cracker puns, she says, the feel-good effect is more probable to be triggered not by the gag itself, but from the response to it.

"The laughter is key. The gag is the terrible 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 ultimate joke?

Probably not, but that has not prevented researchers from trying to.

In 2001, a professor set up a scientific project for the planet's funniest joke.

Over 40,000 jokes submitted, with scores lodged by hundreds of thousands of people globally, he has a clearer understanding than many as to what succeeds and what fails.

The ideal festive cracker joke must be short, he explains.

"They must also be bad gags, puns that make us groan," he adds.

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

"The reason is that if nobody laughs – it's the joke's shortcoming, not yours.

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

"That's a shared experience around the table and I believe it's wonderful."