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"How much did Santa's sled cost? Nothing, 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 company that produces supplies for social events. Its repertoire features Christmas crackers.

The company's owner smiles, nearly sheepishly at the gag. But the joke has been selected and will appear in upcoming crackers.

"The success is gauged by the gag by the number of moans and the intensity of the groans around the table," she explains.

The secret to a good holiday cracker joke is not the same as a good gag per se. It is entirely about the context - in this case, the shared amusement of the Christmas meal with elders, children and possibly neighbours.

"The goal is for the gag to be something that unites the eight-year-old together with the 80-year-old," she states.

The Science Of Shared Laughter

Gathering to enjoy communal amusement is not only ancient, scientists argue, it is probably to be older than humanity.

"Therefore when you are chuckling with others at the Christmas dinner you are dropping into what's very likely a truly primordial mammalian play vocalisation," says a neuroscience expert.

Communal laughter, she says, helps make and maintain social bonds between people.

Scientists have discovered that a lack of these social exchanges can significantly harm both psychological and bodily well-being.

"Those you talk to, and share laughter with, it results in increased levels of 'happy chemical' release," she adds.

These natural chemicals are the brain's "happy chemicals" and are produced both to reduce tension and discomfort and in response to enjoyable activities, such as chuckling with loved ones over a particularly awful festive cracker gag.

"It's not simply chuckling at a foolish joke with a Christmas cracker," she states. "You are in fact doing a lot of the really vital task of making, maintaining the connections you have with the people you love."

What Occurs In the Mind?

But what is actually happening within the mind when we hear a gag?

A tremendous amount occurs in response to humour, it transpires.

Using brain scanning technology, a type of neural imager which shows which parts of the mind are working harder, researchers have been able to chart the regions that receive more blood flow.

Testing entails imaging the brains of healthy participants and then exposing them to a database of humorous words, accompanied by either a neutral sound, or pre-recorded laughter.

"During the study we got a very fascinating activation pattern of neural activity," notes the neuroscientist.

A joke activates not just the areas of the brain in charge of auditory processing and interpreting language, but also brain regions involved in both preparation and starting motion and those linked to sight and recall.

Put these elements as a whole, and people listening to a joke have a complex set of neural reactions that underpin the laughter we hear.

The Contagious Power of Laughter

Researchers found that when a funny phrase is combined with chuckles there is a stronger response in the brain than the identical word when accompanied by a non-emotional sound.

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

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

Amusement, according to the professor, can be contagious.

So what does this mean for the chuckles heard at a Christmas table?

"People laugh more when you know others," she notes, "and you laugh further when you like them or care for them."

When it comes to festive cracker puns, she says, the positive factor is more probable to be caused not by the gag in itself, but from the response to it.

"It's the laughter. The gag is the terrible Christmas cracker pun, and it's just a reason to chuckle as a group."

The Quest for the Ideal Cracker Joke

Is it possible to find the perfect joke?

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

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

Over tens of thousands of gags later, with scores provided by 350,000 participants around the world, he has a better understanding than many as to what succeeds and what does not.

The perfect festive cracker joke needs to be brief, he explains.

"They must also need to be poor jokes, puns that cause us to moan," he continues.

The increasingly "terrible" the gag, 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 holiday cracker jokes is that not one person find them funny.

"It creates a common moment at the gathering and I believe it's lovely."