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

This one-liner is greeted with moans that echo through a warehouse in the capital.

This describes a humor-evaluation meeting with a firm that makes products for social events. Its catalogue includes festive crackers.

The company's founder grins, nearly sheepishly at the joke. But the pun has been selected and will feature in future crackers.

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

The secret to a great holiday cracker joke is not the identical as a stand-up joke per se. It is all about the setting - in this case, the communal amusement of the holiday meal with grandparents, kids and potentially neighbours.

"The goal is for the gag to be a thing that brings the child in harmony with the 80-year-old," she states.

The Science Behind Shared Amusement

Coming together to enjoy shared amusement is not only nothing new, scientists say, it is probably to be pre-human.

"Therefore when you are laughing with others around the Christmas table you are engaging in what's almost certainly a really primordial mammalian play vocalisation," says a professor.

Shared amusement, she says, aids in make and maintain social bonds between individuals.

Scientists have found that a lack of such social exchanges can significantly damage both psychological and bodily well-being.

"Those you talk to, and share laughter with, it leads to increased amounts of endorphin uptake," the professor adds.

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 loved ones over a truly awful festive cracker gag.

"It's not simply chuckling at a foolish joke with a holiday cracker," she says. "You are actually performing a lot of the really vital work of building, preserving the connections you have with those you love."

What Happens Inside the Brain?

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

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

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

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

"In the scanner we got a very interesting activation pattern of activation," notes the neuroscientist.

A gag activates not just the areas of the brain responsible for hearing and interpreting speech, but also brain regions associated with both preparation and starting motion and those involved in vision and memory.

Put all of this as a whole, and individuals listening to a pun have a sophisticated set of brain reactions that underpin the laughter we hear.

The Contagious Power of Laughter

Researchers discovered that when a humorous phrase is combined with chuckles there is a stronger response in the brain than the same word when followed by a non-emotional sound.

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

It indicates we are not just responding to funny jokes, they are reacting to the amusement that accompanies them.

Laughter, says the expert, can be infectious.

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

"You laugh more when you know people," she notes, "and laughter increases further when you are fond of them or love them."

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

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

The Search for the Perfect Cracker Joke

Is it possible to discover the ultimate joke?

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

Years ago, a psychologist established a research search for the planet's most humorous gag.

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

The perfect Christmas cracker joke must be brief, he explains.

"They must also be bad jokes, puns that cause us to groan," he adds.

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

"This is because if no-one laughs – it's the joke's fault, not your own.

"What's interesting about the Christmas cracker puns is that none of us considers them humorous.

"It creates a common moment around the gathering and I think it's lovely."