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

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

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

The company's founder grins, nearly sheepishly at the joke. But the joke has made the cut and will appear 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 explains.

The secret to a good Christmas cracker joke is not the same as a good joke in itself. It is entirely about the setting - in this instance, the shared laughter of the holiday dinner table with elders, kids and potentially friends.

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

The Neuroscience Of Shared Amusement

Gathering to enjoy communal amusement is not only nothing new, scientists argue, it is likely to be pre-human.

"So when you are laughing with others around the Christmas dinner you are engaging in what's very likely a truly ancient mammalian play sound," says a neuroscience expert.

Shared laughter, she says, helps make and maintain social bonds between individuals.

Scientists have found that a lack of such interactions can significantly harm both psychological and bodily health.

"Those you talk to, and laugh with, it results in increased amounts of endorphin release," the professor adds.

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

"You're not just laughing at a foolish joke with a Christmas cracker," she states. "You are actually performing a lot of the truly important task of making, maintaining the connections you have with those you care about."

What Happens Inside the Brain?

But what is actually happening inside the brain when we hear a gag?

An awful lot occurs in reaction to comedy, it turns out.

Employing 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 get more blood flow.

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

"In the scanner we got a very interesting pattern of activation," says the neuroscientist.

A gag stimulates not just the parts of the brain in charge of hearing and interpreting speech, but also neural areas involved in both preparation and initiating motion and those involved in vision and recall.

Put all of this together, and individuals hearing a pun have a complex set of brain responses that underpin the amusement we experience.

The Infectious Power of Chuckles

Scientists found that when a funny phrase is combined 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 employ to move your face into a grin or a laugh," the professor explains.

It indicates we are not just responding to humorous words, they are reacting to the amusement that accompanies them.

Amusement, according to the professor, can be contagious.

So what does this imply for the chuckles found around a holiday table?

"You laugh harder when you know people," she says, "and you laugh further when you like them or care for them."

When it comes to Christmas cracker puns, she says, the positive factor is more probable to be triggered not by the gag itself, but from the response to it.

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

The Quest for the Perfect Festive Pun

Will we ever find the perfect gag?

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

In 2001, a professor established a research project for the world's funniest joke.

More than tens of thousands of gags later, with ratings provided by hundreds of thousands of people globally, he has a clearer understanding than many as to what succeeds and what does not.

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

"They must also need to be bad gags, puns that cause us to groan," he adds.

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

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

"What's interesting about the holiday cracker jokes is that none of us find them humorous.

"It creates a common experience around the table and I believe it's lovely."