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"How much did Santa's sled cost? Zero, it was on the house."

This quip is met by moans that echo through a storage facility in London.

This describes a joke-testing meeting with a company that makes products for social events. Its repertoire features festive crackers.

The company's owner grins, almost apologetically at the gag. But the pun has made the cut and will appear in future crackers.

"The success is gauged by the joke by the volume of moans and the loudness of the groans around the table," she says.

The secret to a good Christmas cracker joke is not the same as a good gag in itself. It is all about the setting - in this case, the communal laughter of the Christmas dinner table with elders, children and potentially neighbours.

"The goal is for the gag to be a thing that brings the eight-year-old in harmony with the grandparent," she states.

The Neuroscience Of Communal Amusement

Gathering to enjoy shared laughter is not only nothing new, scientists say, it is probably to be older than humanity.

"Therefore when you are laughing with people at the holiday dinner you are dropping into what's almost certainly a really primordial mammalian social sound," explains a professor.

Communal amusement, she explains, helps forge and strengthen social connections between individuals.

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

"The people you converse with, and laugh with, it leads to increased amounts of endorphin uptake," she adds.

These natural chemicals are the brain's "happy chemicals" and are produced both to alleviate tension and discomfort and in response to pleasurable experiences, such as chuckling with friends over a truly terrible Christmas cracker joke.

"It's not simply laughing at a silly joke with a Christmas cracker," she states. "You are actually doing a lot of the really important work of building, preserving the connections you have with those you care about."

What Occurs Inside the Mind?

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

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

Employing brain scanning technology, a type of neural imager which indicates which parts of the brain are working harder, scientists have been able to chart the regions that receive more blood flow.

Testing entails scanning the brains of volunteer participants and then subjecting them to a database of funny words, paired with either a non-emotional sound, or pre-recorded chuckles.

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

A gag stimulates not just the areas of the mind in charge of auditory processing and interpreting speech, but also neural regions involved in both planning and initiating motion and those linked to sight and recall.

Put all of this as a whole, and people listening to a joke have a complex series of brain responses that underpin the laughter we experience.

The Infectious Power of Chuckles

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

"This activation occurred in parts of the brain that you would employ to contort your face into a smile or a chuckle," she explains.

It means we are not just responding to humorous jokes, they are responding to the amusement that accompanies them.

Laughter, according to the expert, can be contagious.

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

"You laugh harder when you are familiar with people," she says, "and you laugh more when you are fond of them or love them."

When it comes to Christmas cracker puns, she explains, the feel-good effect is more probable to be triggered not by the joke in itself, but from the reaction to it.

"The laughter is key. The gag is the terrible Christmas cracker pun, and it's just a reason to laugh together."

The Quest for the Ideal Cracker Joke

Will we ever discover the perfect gag?

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

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

More than tens of thousands of jokes later, with ratings lodged by 350,000 people around the world, he has a clearer idea than most as to what succeeds and what does not.

The ideal Christmas cracker pun needs to be short, he says.

"But they also need to be poor gags, jokes that make us moan," he adds.

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

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

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

"It creates a shared experience around the table and I think it's lovely."