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

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

This describes a humor-evaluation session with a company that makes products for social events. Its repertoire includes festive crackers.

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

"The success is gauged by the gag by the volume of moans and the intensity of the groans around the table," the founder says.

The key to a great Christmas cracker joke is not the identical as a stand-up gag per se. It is all about the setting - in this case, the shared laughter of the holiday meal with elders, children and possibly neighbours.

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

The Neuroscience Of Shared Laughter

Gathering to experience shared amusement is not only nothing new, scientists argue, it is likely to be older than humanity.

"So when you are laughing with others around the Christmas dinner you are dropping into what's very likely a truly ancient mammal social sound," says a professor.

Shared laughter, she says, aids in forge and strengthen social bonds between people.

Researchers have discovered that a absence of these social exchanges can significantly damage mental and physical well-being.

"The people you talk to, and share laughter with, it leads to increased levels of 'happy chemical' uptake," the professor continues.

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

"You're not just laughing at a foolish joke with a Christmas cracker," the expert states. "You are actually performing a lot of the truly vital task of building, preserving the social bonds you have with those you care about."

What Happens In the Mind?

But what is truly happening inside the brain when we hear a joke?

An awful lot occurs in reaction to comedy, it transpires.

Employing functional magnetic resonance imaging (fMRI), a kind of neural imager which shows which areas of the mind are more active, researchers have been able to map the areas that receive more blood flow.

Testing involves scanning the brains of healthy participants and then subjecting them to a collection of humorous words, paired with either a neutral sound, or recorded laughter.

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

A joke stimulates not just the parts of the brain responsible for hearing and understanding language, but also neural areas involved in both planning and initiating movement and those linked to vision and memory.

Combine these elements as a whole, and people listening to a pun have a sophisticated series of neural reactions that underpin the amusement we hear.

The Contagious Nature of Chuckles

Researchers found that when a funny word is paired with chuckles there is a greater reaction in the brain than the same phrase when accompanied by a neutral sound.

"This was in areas of the mind that you would use to contort your face into a smile or a laugh," she explains.

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

Amusement, according to the expert, can be contagious.

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

"People laugh more when you know people," she says, "and you laugh further when you are fond of them or love them."

When it comes to Christmas cracker puns, she says, the positive effect is more likely to be triggered not by the gag 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 Cracker Joke

Will we ever find the perfect joke?

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

In 2001, a professor set up a scientific search for the world's funniest joke.

Over 40,000 gags later, with ratings provided by hundreds of thousands of participants globally, he has a clearer understanding than many as to what works and what fails.

The ideal Christmas cracker pun must be brief, he explains.

"But they also be bad gags, jokes that cause us to groan," he adds.

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

"The reason is that if nobody finds it funny – it's the joke's fault, not your own.

"What's interesting about the Christmas cracker puns is that not one person considers them humorous.

"That's a shared experience at the gathering and I think it's wonderful."