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

This joke is met by groans that resonate through a storage facility in London.

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

The company's founder smiles, nearly apologetically at the gag. But the pun has made the cut and will feature in future crackers.

"The success is gauged by the joke by the volume of moans and the intensity of the groans at the table," the founder explains.

The key to a great holiday cracker joke is not the same as a good joke per se. It is all about the context - in this case, the communal laughter of the holiday dinner table with elders, kids and possibly neighbours.

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

The Neuroscience Behind Communal Laughter

Coming together to experience shared laughter is not only nothing new, scientists say, it is likely to be older than humanity.

"Therefore when you are chuckling with others around the Christmas table you are engaging in what's almost certainly a really ancient mammalian social vocalisation," explains a neuroscience expert.

Communal laughter, she explains, helps make and maintain social bonds between individuals.

Scientists have discovered that a absence of such interactions can significantly harm both psychological and bodily well-being.

"The people you converse with, and share laughter with, it leads to increased amounts of endorphin release," the professor adds.

These natural chemicals are the brain's "happy chemicals" and are produced both to reduce tension and discomfort and in response to pleasurable activities, such as laughing with loved ones over a particularly awful Christmas cracker gag.

"You're not just chuckling at a silly joke with a holiday cracker," she says. "You are actually doing a lot of the really important work of building, preserving the connections you have with the people you care about."

What Occurs In the Brain?

But what is truly happening within the mind when we listen to a gag?

An awful lot happens in reaction to humour, it turns out.

Using brain scanning technology, a type of neural imager which indicates which areas of the brain are working harder, researchers have been able to chart the areas that get more blood.

The research entails scanning the minds of healthy participants and then exposing them to a database of humorous phrases, paired with either a neutral sound, or recorded laughter.

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

A joke activates not just the areas of the mind responsible for auditory processing and understanding language, but also brain areas involved in both preparation and starting motion and those involved in vision and recall.

Combine all of this as a whole, and individuals listening to a joke have a complex series of neural responses that underpin the laughter we experience.

The Contagious Nature of Laughter

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

"This was in areas of the brain that you would employ to contort your expression into a smile or a laugh," she explains.

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

Amusement, according to the expert, can be infectious.

So what does this mean for the laughter found around a holiday gathering?

"You laugh harder when you know 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 explains, the positive factor is more probable to be triggered not by the joke itself, but from the reaction to it.

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

The Search for the Ideal Cracker Joke

Is it possible to find the ultimate joke?

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

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

More than tens of thousands of gags submitted, with ratings lodged by 350,000 participants around the world, he has a better understanding than most as to what works and what fails.

The perfect festive cracker joke needs to be brief, he says.

"They must also be poor gags, puns that cause us to moan," he continues.

The increasingly "awful" the gag, he states the more effective.

"The reason is that if no-one laughs – it's the gag's fault, not your own.

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

"It creates a common moment at the table and I think it's wonderful."