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"How much did Santa's sleigh cost? Nothing, it was on the house."

This quip is greeted with groans that resonate through a warehouse in the capital.

We're at a humor-evaluation session with a company that makes products for gatherings. Its repertoire features festive crackers.

The firm's owner grins, almost sheepishly at the joke. But the pun has made the cut and will appear in future crackers.

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

The secret to a good Christmas cracker pun is not the identical as a stand-up joke per se. It is all about the context - in this case, the shared laughter of the Christmas meal with grandparents, kids and possibly neighbours.

"You want the gag to be a thing that unites the child together with the grandparent," she states.

The Neuroscience Behind Shared Laughter

Gathering to experience shared laughter is not only ancient, experts argue, it is probably to be pre-human.

"So when you are laughing with others at the holiday dinner you are dropping into what's very likely a really ancient mammal play vocalisation," explains a neuroscience expert.

Shared laughter, she says, aids in make and maintain social connections between individuals.

Researchers have found that a lack of such social exchanges can significantly damage mental and physical health.

"The people you converse with, and share laughter with, it results in increased levels of endorphin release," the professor continues.

Endorphins are the brain's "feel-good compounds" and are released both to reduce tension and discomfort and in response to pleasurable activities, such as chuckling with friends over a truly terrible festive cracker gag.

"You're not just laughing at a foolish joke with a Christmas cracker," she states. "You are in fact doing a lot of the truly important task of building, preserving the connections you have with those you care about."

Which Occurs In the Brain?

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

An awful lot occurs in response to humour, it transpires.

Employing brain scanning technology, a type of brain scanner which shows which areas of the brain are more active, researchers have been able to map the regions that receive more blood.

Testing involves imaging the brains of healthy subjects and then subjecting them to a collection of humorous words, accompanied by either a neutral sound, or pre-recorded chuckles.

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

A gag stimulates not just the areas of the mind responsible for auditory processing and understanding language, but also neural regions involved in both preparation and starting motion and those linked to sight and memory.

Put all of this as a whole, and people listening to a pun have a complex series of neural responses that underpin the laughter we hear.

The Contagious Power of Chuckles

Scientists discovered that when a humorous phrase is paired with laughter there is a greater response in the brain than the same word when followed by a neutral sound.

"This was in parts of the mind that you would employ to move your expression into a smile or a laugh," the professor says.

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

Laughter, according to the professor, can be contagious.

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

"People laugh more when you are familiar with people," she notes, "and you laugh further when you are fond of them or love them."

When it comes to festive cracker puns, she says, the feel-good factor is more likely to be caused not by the joke itself, but from the response to it.

"It's the laughter. The joke is the terrible holiday cracker pun, and it's just a reason to laugh together."

The Search for the Perfect Festive Pun

Is it possible to find the ultimate gag?

Probably not, but that has not stopped experts from trying to.

In 2001, a professor set up a scientific project for the world's funniest gag.

More than tens of thousands of gags later, with scores provided by hundreds of thousands of participants around the world, he has a better idea than most as to what works and what does not.

The ideal festive cracker pun needs to be brief, he says.

"But they also need to be bad jokes, jokes that make us groan," he adds.

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

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

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

"That's a common experience around the table and I think it's wonderful."