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"How much did Santa's sleigh cost? Zero, it was on the house."

This one-liner is met by groans that resonate through a warehouse in the capital.

We're at a humor-evaluation meeting with a company that produces supplies for social events. Its repertoire features festive crackers.

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

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

The key to a great Christmas cracker joke is not the identical as a good gag per se. It is entirely about the context - in this case, the shared laughter of the Christmas dinner table with elders, children and possibly neighbours.

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

The Science Behind Shared Amusement

Gathering to enjoy shared laughter is not only nothing new, scientists say, it is probably to be pre-human.

"Therefore when you are chuckling with others around the Christmas table you are dropping into what's almost certainly a really primordial mammalian play sound," explains a neuroscience expert.

Shared amusement, she explains, helps make and maintain social connections between individuals.

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

"The people you converse with, and laugh with, it leads to enhanced levels of endorphin uptake," she adds.

These natural chemicals are the body's "feel-good compounds" and are released both to reduce stress and pain and in reaction to pleasurable activities, such as chuckling with friends over a particularly awful festive cracker joke.

"It's not simply chuckling at a silly pun with a Christmas cracker," the expert states. "You are in fact doing a lot of the really vital task of making, maintaining the social bonds you have with the people you love."

Which Happens Inside the Mind?

But what is actually happening inside the mind when we hear a joke?

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

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

The research involves imaging the minds of volunteer participants and then exposing them to a database of funny words, paired with either a non-emotional sound, or pre-recorded laughter.

"During the study we got a really fascinating activation pattern of activation," notes the neuroscientist.

A gag stimulates not just the areas of the mind in charge of auditory processing and interpreting speech, but also neural regions associated with both planning and initiating movement and those linked to vision and recall.

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

The Contagious Power of Laughter

Researchers discovered that when a funny word is combined with chuckles there is a stronger reaction in the brain than the identical word when followed by a neutral sound.

"This was in areas of the brain that you would employ to move your face into a smile or a laugh," she says.

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

Laughter, according to the professor, can be infectious.

So what does this imply for the chuckles found at a Christmas table?

"People laugh harder when you know people," she says, "and laughter increases more when you are fond of them or care for them."

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

"It's the laughter. The joke is the terrible holiday cracker pun, and it's just a pretext to chuckle as a group."

The Search for the Ideal Festive Pun

Is it possible to find the perfect gag?

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

In 2001, a psychologist established a scientific project for the world's most humorous gag.

More than tens of thousands of gags submitted, with ratings provided by 350,000 people around the world, he has a better understanding than many as to what works and what does not.

The perfect Christmas cracker joke needs to be brief, he says.

"But they also need to be bad jokes, puns that make us moan," he continues.

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

"This is because if nobody laughs – it's the joke's shortcoming, not your own.

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

"That's a common moment around the table and I think it's lovely."