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

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

This describes a humor-evaluation meeting with a company that makes supplies for gatherings. Its repertoire includes festive crackers.

The company's owner smiles, nearly apologetically at the joke. But the joke has been selected and will feature in future crackers.

"You measure the joke by the number of groans and the loudness of the groans around the table," the founder explains.

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

"You want the joke to be something that brings the eight-year-old together with the 80-year-old," she adds.

The Science Behind Communal Amusement

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

"Therefore when you are chuckling with people around the holiday dinner you are dropping into what's almost certainly a really ancient mammal social sound," says a professor.

Shared amusement, she says, helps forge and strengthen social connections between people.

Scientists have discovered that a lack of these interactions can seriously harm mental and physical well-being.

"Those you talk to, and laugh with, it results in enhanced levels of endorphin uptake," she adds.

Endorphins are the brain's "happy chemicals" and are produced both to alleviate tension and discomfort and in reaction to enjoyable activities, such as laughing with friends over a truly terrible Christmas cracker gag.

"You're not just laughing at a foolish joke with a Christmas cracker," she states. "You are in fact doing a lot of the truly vital work of building, preserving the social bonds you have with those you care about."

What Happens Inside the Mind?

But what is actually taking place inside the brain when we hear a joke?

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

Employing functional magnetic resonance imaging (fMRI), a kind of brain scanner which shows which parts of the brain are more active, scientists have been able to map the areas that get more blood flow.

Testing entails imaging the brains of healthy subjects and then subjecting them to a database of funny words, accompanied by either a neutral sound, or pre-recorded chuckles.

"In the scanner we observed a very fascinating pattern of activation," says the neuroscientist.

A gag activates not just the areas of the brain responsible for auditory processing and understanding language, but also brain areas associated with both preparation and starting movement and those linked to sight and memory.

Combine all of this as a whole, and individuals hearing a joke have a complex series of brain reactions that support the amusement 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 identical phrase when followed by a neutral sound.

"This activation occurred in areas of the brain that you would employ to contort your expression into a smile or a chuckle," the professor says.

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

Amusement, according to the expert, can be contagious.

So what does this imply for the laughter heard at a holiday gathering?

"People laugh harder when you are familiar with others," she says, "and laughter increases further when you like them or care for them."

When it comes to Christmas cracker puns, she says, the feel-good factor is more likely to be caused not by the joke itself, but from the response to it.

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

The Search for the Perfect Festive Pun

Will we ever find the perfect joke?

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

Years ago, a professor set up a research project for the planet's most humorous joke.

More than tens of thousands of gags later, with scores provided by 350,000 people globally, he has a better idea than most as to what works and what does not.

The ideal Christmas cracker joke needs to be brief, he says.

"But they also be bad jokes, jokes that make us groan," he continues.

The increasingly "terrible" the joke, he states the more effective.

"This is because if nobody laughs – it's the gag's shortcoming, not yours.

"The fascinating part about the holiday cracker puns is that none of us find them humorous.

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