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

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

This describes a humor-evaluation session with a firm that produces supplies for gatherings. Its catalogue features Christmas crackers.

The firm's founder grins, almost apologetically at the joke. But the joke has been selected and will appear in future crackers.

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

The secret to a good holiday cracker pun is not the same as a stand-up gag in itself. It is all about the setting - in this instance, the communal laughter of the holiday dinner table with elders, kids and potentially friends.

"You want the gag to be something that brings the eight-year-old together with the grandparent," she states.

The Science Behind Shared Laughter

Gathering to enjoy communal amusement is not only nothing new, experts argue, it is probably to be older than humanity.

"Therefore when you are laughing with people at the Christmas table you are dropping into what's almost certainly a really primordial mammalian social vocalisation," explains a neuroscience expert.

Communal laughter, she explains, helps forge and strengthen social bonds between people.

Researchers have discovered that a lack of such social exchanges can seriously damage both psychological and bodily well-being.

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

These natural chemicals are the brain's "happy chemicals" and are produced both to reduce stress and pain and in reaction to pleasurable experiences, such as laughing with friends over a particularly terrible festive cracker gag.

"It's not simply laughing at a foolish joke with a holiday cracker," she says. "You are in fact performing a lot of the really important task of building, preserving the connections you have with the people you care about."

Which Happens In the Brain?

But what is actually taking place inside the mind when we hear 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 areas of the brain are working harder, researchers have been able to map the regions that receive more blood.

Testing entails imaging the brains of healthy subjects and then exposing them to a database of funny phrases, paired with either a non-emotional sound, or pre-recorded laughter.

"In the scanner we observed a really fascinating pattern of neural activity," notes the professor.

A joke stimulates not just the parts of the brain responsible for auditory processing and interpreting language, but also brain regions associated with both preparation and initiating movement and those linked to vision and recall.

Combine all of this together, and people hearing a joke have a sophisticated set of neural reactions that support the laughter we experience.

The Contagious Nature of Chuckles

Scientists discovered that when a humorous word is paired with chuckles there is a greater response in the mind than the same phrase when followed by a non-emotional sound.

"This activation occurred in parts of the brain that you would use to move your face into a grin or a laugh," the professor explains.

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

Amusement, according to the professor, can be infectious.

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

"People laugh more when you are familiar with others," she notes, "and you laugh more when you like them or love them."

When it comes to festive cracker jokes, she explains, the feel-good factor is more likely to be caused not by the gag itself, but from the reaction to it.

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

The Search for the Perfect Cracker Joke

Will we ever find the ultimate gag?

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

Years ago, a psychologist set up a research project for the planet's funniest joke.

More than 40,000 jokes submitted, with ratings provided by hundreds of thousands of participants around the world, he has a better understanding than most as to what succeeds and what does not.

The ideal Christmas cracker joke must be short, he explains.

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

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

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

"The fascinating part about the Christmas cracker jokes is that not one person find them funny.

"It creates a shared experience at the gathering and I believe it's lovely."