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

This one-liner is met by moans that echo through a storage facility in the capital.

We're at a joke-testing meeting with a firm that makes supplies for gatherings. Its catalogue features festive crackers.

The firm's founder grins, almost sheepishly at the joke. But the pun has been selected and will appear in future crackers.

"You measure the joke by the volume of groans and the intensity of the groans at the table," the founder says.

The secret to a good holiday cracker joke is not the same as a stand-up gag in itself. It is all about the setting - in this instance, the shared amusement of the Christmas meal with elders, kids and potentially friends.

"You want the gag to be a thing that brings the child in harmony with the 80-year-old," she states.

The Neuroscience Behind Shared Amusement

Gathering to experience communal laughter is not only nothing new, experts say, it is probably to be pre-human.

"So when you are laughing with others at the holiday dinner you are dropping into what's almost certainly a really ancient mammalian social vocalisation," says a neuroscience expert.

Communal amusement, she explains, helps make and maintain social bonds between individuals.

Scientists have discovered that a absence of such social exchanges can significantly damage mental and physical well-being.

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

These natural chemicals are the brain's "feel-good compounds" and are produced both to alleviate stress and pain and in response to pleasurable experiences, such as laughing with loved ones over a particularly awful festive cracker joke.

"You're not just chuckling at a silly joke with a holiday cracker," she says. "You are in fact doing a lot of the truly vital task of building, preserving the connections you have with those you care about."

Which Happens Inside the Mind?

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

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

Using brain scanning technology, a kind of neural imager which shows which areas of the mind are more active, scientists have been able to map the regions that receive more blood flow.

Testing entails scanning the brains of healthy participants and then subjecting them to a collection of funny words, paired with either a non-emotional sound, or recorded laughter.

"During the study we observed a really fascinating pattern of neural activity," notes the neuroscientist.

A joke stimulates not just the areas of the brain in charge of auditory processing and understanding language, but also brain regions involved in both preparation and starting motion and those involved in vision and memory.

Combine all of this together, and people listening to a pun have a complex set of brain responses that support the laughter we hear.

The Infectious Nature of Laughter

Scientists discovered that when a funny word is combined with chuckles there is a stronger reaction in the mind than the identical word when accompanied by a neutral sound.

"This activation occurred in parts of the brain that you would use to contort your face into a smile or a chuckle," she explains.

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

Laughter, says the expert, can be contagious.

So what does this mean for the laughter found at a Christmas table?

"You laugh harder when you know people," she notes, "and you laugh further when you are fond of them or love them."

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

"The laughter is key. The gag is the terrible holiday cracker pun, and it's just a pretext to laugh as a group."

The Quest for the Ideal Cracker Joke

Is it possible to find the ultimate gag?

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

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

Over tens of thousands of gags later, with scores provided by 350,000 participants globally, he has a better understanding than most as to what works and what does not.

The perfect festive cracker pun must be short, he explains.

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

The more "awful" the joke, he states the better.

"This is because if nobody laughs – it's the gag's shortcoming, not yours.

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

"That's a shared experience at the gathering and I think it's wonderful."