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

This quip is greeted with groans that resonate through a warehouse in London.

We're at a humor-evaluation session with a firm that makes supplies for gatherings. Its catalogue includes Christmas crackers.

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

"You measure the gag by the volume of groans and the loudness of the groans around the table," the founder explains.

The key to a great Christmas cracker joke is not the identical as a stand-up gag in itself. It is all about the context - in this instance, the shared laughter of the holiday dinner table with grandparents, children and potentially neighbours.

"The goal is for the gag to be a thing that unites the child in harmony with the grandparent," she states.

The Science Of Shared Laughter

Gathering to experience communal laughter is not only ancient, experts argue, it is likely to be pre-human.

"So when you are chuckling with people around the Christmas dinner you are engaging in what's almost certainly a really ancient mammalian social sound," says a neuroscience expert.

Shared amusement, she explains, helps make and maintain social bonds between individuals.

Researchers have found that a absence of these interactions can significantly damage mental and physical health.

"Those you converse with, and laugh with, it results in enhanced levels of endorphin release," the professor continues.

These natural chemicals are the brain's "happy chemicals" and are released both to reduce tension and discomfort and in reaction to pleasurable experiences, such as laughing with friends over a truly awful Christmas cracker gag.

"You're not just chuckling at a foolish joke with a holiday cracker," she states. "You are actually performing a lot of the really vital task of building, preserving the social bonds you have with the people you care about."

What Happens Inside the Brain?

But what is actually happening within the mind when we listen to a joke?

A tremendous amount happens in response to humour, it transpires.

Using functional magnetic resonance imaging (fMRI), a type of brain scanner which shows which areas of the brain are more active, researchers have been able to map the areas that get more blood.

Testing entails scanning the brains of healthy subjects and then exposing them to a collection of funny phrases, paired with either a non-emotional sound, or pre-recorded laughter.

"During the study we got a very fascinating pattern of activation," notes the neuroscientist.

A joke activates not just the parts of the brain in charge of auditory processing and interpreting speech, but also brain regions involved in both planning and initiating movement and those linked to vision and memory.

Put all of this together, and individuals listening to a pun have a sophisticated series of neural reactions that underpin the amusement we hear.

The Contagious Nature of Chuckles

Scientists found that when a funny word is paired with chuckles there is a stronger response in the brain than the identical phrase when accompanied by a neutral sound.

"This activation occurred in areas of the mind that you would use to move your expression into a grin or a chuckle," the professor explains.

It means people are not just reacting to funny words, they are reacting to the laughter that follows them.

Amusement, according to the expert, can be infectious.

So what does this mean for the chuckles heard around a Christmas gathering?

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

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

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

The Quest for the Perfect Cracker Joke

Is it possible to find the ultimate gag?

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

In 2001, a psychologist established a research project for the planet's funniest gag.

Over 40,000 gags later, with scores lodged by 350,000 people globally, he has a clearer idea than most as to what succeeds and what fails.

The perfect festive cracker pun needs to be short, he says.

"They must also be bad gags, puns that make us moan," he continues.

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

"The reason is that if nobody finds it funny – it's the gag's shortcoming, not yours.

"What's interesting about the holiday cracker jokes is that not one person find them funny.

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