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"What was the price did Father Christmas's sled cost? Zero, it was on the house."

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

We're at a joke-testing meeting with a firm that makes supplies for gatherings. Its repertoire includes Christmas crackers.

The company's founder smiles, almost sheepishly at the gag. But the joke has made the cut and will appear in future crackers.

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

The key to a good Christmas cracker joke is not the same as a stand-up joke per se. It is entirely about the setting - in this case, the communal amusement of the holiday dinner table with elders, kids and possibly friends.

"You want the joke to be something that unites the eight-year-old together with the grandparent," she adds.

The Science Of Communal Laughter

Gathering to enjoy shared laughter is not only nothing new, scientists say, it is probably to be pre-human.

"So when you are chuckling with people around the holiday table you are engaging in what's very likely a truly ancient mammal social vocalisation," explains a professor.

Shared laughter, she says, helps forge and strengthen social connections between individuals.

Scientists have found that a absence of these interactions can significantly damage mental and physical well-being.

"The people you converse with, and laugh with, it leads to increased amounts of endorphin uptake," the professor adds.

These natural chemicals are the body's "feel-good compounds" and are released both to reduce stress and pain and in response to pleasurable experiences, such as laughing with loved ones over a particularly terrible Christmas cracker joke.

"It's not simply laughing at a foolish joke with a holiday cracker," the expert says. "You are actually performing a lot of the really vital work of making, maintaining the social bonds you have with those you love."

Which Occurs Inside the Mind?

But what is actually happening inside the mind when we hear a joke?

An awful lot occurs in response to humour, it turns out.

Employing functional magnetic resonance imaging (fMRI), a type of neural imager which indicates which parts of the mind are more active, researchers have been able to chart the areas that receive more blood flow.

Testing entails scanning the minds of healthy subjects and then subjecting them to a database of funny phrases, accompanied by either a neutral sound, or recorded chuckles.

"During the study we got a really fascinating pattern of neural activity," says the professor.

A gag activates not just the areas of the brain responsible for auditory processing and understanding language, but also neural regions involved in both planning and initiating movement and those linked to sight and recall.

Put these elements as a whole, and people hearing a joke have a complex set of neural reactions that support the amusement we experience.

The Contagious Nature of Chuckles

Researchers found that when a funny phrase is paired with laughter there is a stronger reaction in the brain than the identical phrase when followed by a neutral sound.

"This activation occurred in parts of the mind that you would use to contort your expression into a grin or a laugh," she says.

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

Laughter, according to the expert, can be infectious.

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

"People laugh more when you are familiar with others," she says, "and you laugh more when you are fond of them or care for them."

When it comes to festive cracker puns, she says, the positive effect is more probable to be triggered not by the joke in itself, but from the response to it.

"It's the laughter. The joke is the terrible Christmas cracker joke, and it's just a pretext to chuckle as a group."

The Quest for the Perfect Cracker Joke

Will we ever find the ultimate gag?

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

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

More than tens of thousands of gags submitted, with ratings lodged by 350,000 people around the world, he has a clearer idea than many as to what works and what fails.

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

"But they also need to be poor gags, jokes that make us groan," he adds.

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

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

"The fascinating part about the Christmas cracker puns is that none of us considers them funny.

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