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

This joke is met by groans that resonate through a warehouse in the capital.

We're at a joke-testing meeting with a company that makes products for social events. Its catalogue includes festive crackers.

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

"The success is gauged by the gag by the volume of moans and the loudness of the groans at the table," she says.

The secret to a good holiday cracker joke is not the identical as a good gag in itself. It is all about the context - in this case, the shared laughter of the Christmas dinner table with elders, kids and potentially neighbours.

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

The Neuroscience Behind Communal Amusement

Gathering to experience shared laughter is not only ancient, scientists argue, it is likely to be older than humanity.

"So when you are laughing with others at the holiday table you are engaging in what's very likely a really ancient mammal social sound," says a neuroscience expert.

Shared laughter, she explains, helps make and maintain social bonds between individuals.

Scientists have found that a absence of such interactions can seriously harm both psychological and bodily health.

"The people you talk to, and share laughter with, it leads to increased amounts of endorphin uptake," she continues.

These natural chemicals are the brain's "happy chemicals" and are produced both to alleviate stress and pain and in reaction to enjoyable experiences, such as chuckling with friends over a truly terrible festive cracker gag.

"You're not just chuckling at a foolish joke with a holiday cracker," she says. "You are in fact doing a lot of the really important work of building, preserving the social bonds you have with the people you love."

What Happens In the Mind?

But what is actually taking place within the brain when we hear a joke?

An awful lot occurs in reaction to humour, it turns out.

Using brain scanning technology, a kind of neural imager which indicates which parts of the mind are more active, researchers have been able to chart the regions that receive more blood.

The research entails scanning the brains of healthy participants and then subjecting them to a database of humorous words, accompanied by either a non-emotional sound, or pre-recorded chuckles.

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

A gag stimulates not just the areas of the brain in charge of hearing and understanding speech, but also brain regions involved in both planning and starting movement and those linked to sight and memory.

Combine these elements as a whole, and people hearing a pun have a complex set of brain responses that underpin the amusement we hear.

The Contagious Nature of Laughter

Scientists found that when a funny phrase is combined with laughter there is a stronger response in the mind than the identical phrase when followed by a neutral sound.

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

It means people are not just responding to funny jokes, they are responding to the laughter that accompanies them.

Laughter, says the expert, can be infectious.

So what does this imply for the laughter heard around a holiday table?

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

When it comes to Christmas cracker puns, she says, the feel-good effect is more probable to be triggered not by the gag itself, but from the response to it.

"The laughter is key. The joke is the terrible Christmas cracker pun, and it's just a pretext to chuckle together."

The Quest for the Perfect Cracker Joke

Is it possible to find the perfect gag?

Probably not, but that has not prevented researchers from trying to.

In 2001, a psychologist established a scientific project for the world's funniest joke.

More than 40,000 jokes later, with scores lodged by 350,000 participants around the world, he has a better idea than most as to what works and what fails.

The ideal Christmas cracker joke needs to be short, he explains.

"But they also be bad gags, jokes that make us moan," he continues.

The increasingly "awful" the gag, he says the more effective.

"This is because if no-one laughs – it's the joke's fault, not yours.

"What's interesting about the Christmas cracker jokes is that not one person considers them funny.

"It creates a shared experience around the table and I believe it's wonderful."