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

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

We're at a humor-evaluation meeting with a firm that produces products for gatherings. Its repertoire includes Christmas crackers.

The firm's owner smiles, nearly apologetically at the joke. But the joke has made the cut and will appear in upcoming crackers.

"The success is gauged by the joke by the volume of groans and the intensity of the groans at the table," the founder explains.

The secret to a good holiday cracker joke is not the same as a good gag in itself. It is entirely about the context - in this instance, the communal laughter of the holiday dinner table with grandparents, kids and possibly neighbours.

"You want the joke to be a thing that unites the child in harmony with the 80-year-old," she adds.

The Neuroscience Behind Shared Amusement

Gathering to enjoy shared amusement is not only ancient, experts argue, it is likely to be older than humanity.

"Therefore when you are laughing with people around the holiday dinner you are engaging in what's very likely a really primordial mammalian play vocalisation," says a professor.

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

Scientists have discovered that a lack of such interactions can seriously damage both psychological and bodily well-being.

"The people you talk to, and laugh with, it leads to enhanced amounts of endorphin release," the professor continues.

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

"You're not just laughing at a silly joke with a holiday cracker," the expert says. "You are actually performing a lot of the really important work of making, maintaining the social bonds you have with those you care about."

What Happens Inside the Mind?

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

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

Using functional magnetic resonance imaging (fMRI), a kind of neural imager which shows which parts of the brain are more active, scientists have been able to chart the regions that get more blood flow.

Testing entails scanning the minds of healthy subjects and then subjecting them to a collection of humorous phrases, accompanied by either a non-emotional sound, or pre-recorded laughter.

"In the scanner we got a really interesting pattern of neural activity," notes the professor.

A joke stimulates not just the areas of the brain responsible for auditory processing and understanding language, but also brain regions involved in both preparation and initiating motion and those involved in vision and recall.

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

The Infectious Nature of Chuckles

Researchers found that when a funny phrase is combined with chuckles there is a greater reaction in the mind than the identical phrase when accompanied by a non-emotional sound.

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

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

Amusement, according to the expert, can be infectious.

So what does this mean for the laughter heard at a Christmas gathering?

"People laugh harder when you are familiar with people," she notes, "and you laugh further when you like them or care for them."

When it comes to Christmas cracker puns, she explains, the positive effect is more probable to be triggered not by the gag in itself, but from the reaction to it.

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

The Quest for the Perfect Festive Pun

Will we ever discover the ultimate gag?

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

Years ago, a psychologist established a research search for the world's most humorous joke.

Over 40,000 gags submitted, with scores provided by 350,000 participants around the world, he has a clearer idea than most as to what succeeds and what does not.

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

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

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

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

"What's interesting about the holiday cracker puns is that not one person find them funny.

"That's a common moment around the gathering and I think it's wonderful."