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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 the capital.

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

The company's founder grins, almost apologetically at the joke. But the joke has made the cut and will appear in upcoming crackers.

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

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

"The goal is for the gag to be something that unites the child in harmony with the grandparent," she adds.

The Science Behind Communal Laughter

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

"So when you are laughing with people at the Christmas table you are dropping into what's very likely a truly primordial mammalian play vocalisation," explains a neuroscience expert.

Shared laughter, she explains, helps forge and strengthen social connections between people.

Researchers have found that a lack of these interactions can significantly damage both psychological and bodily health.

"The people you converse with, and laugh with, it leads to enhanced levels of 'happy chemical' release," she adds.

Endorphins are the body's "happy chemicals" and are released both to reduce stress and pain and in reaction to pleasurable activities, such as laughing with loved ones over a particularly awful Christmas cracker gag.

"You're not just chuckling at a foolish pun with a holiday cracker," the expert says. "You are in fact performing a lot of the truly vital task of building, preserving the social bonds you have with those you care about."

What Happens Inside the Brain?

But what is truly taking place inside the brain when we listen to a gag?

An awful lot happens in response to humour, it transpires.

Employing brain scanning technology, a kind of brain scanner which indicates which parts of the mind are working harder, scientists have been able to chart the regions that get more blood.

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

"In the scanner we got a very interesting pattern of neural activity," notes the professor.

A gag activates not just the areas of the brain responsible for hearing and interpreting speech, but also brain regions associated with both preparation and initiating motion and those involved in vision and memory.

Put all of this as a whole, and individuals listening to a joke have a sophisticated series of neural responses that underpin the laughter we hear.

The Contagious Power of Laughter

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

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

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

Laughter, according to the expert, can be contagious.

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

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

When it comes to Christmas cracker puns, she explains, the positive factor is more likely to be triggered not by the gag itself, but from the reaction to it.

"It's the laughter. The joke is the dreadful holiday cracker joke, and it's just a reason to chuckle together."

The Quest for the Perfect Cracker Joke

Will we ever discover the ultimate joke?

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

In 2001, a psychologist set up a scientific search for the world's most humorous joke.

More than 40,000 gags later, with scores lodged by hundreds of thousands of people globally, he has a clearer idea than most as to what succeeds and what fails.

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

"They must also be bad jokes, jokes that cause us to moan," he adds.

The more "terrible" the joke, he says the better.

"This is because if no-one laughs – it's the gag's shortcoming, not yours.

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

"It creates a common moment at the gathering and I believe it's lovely."