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

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

We're at a humor-evaluation session with a company that makes products for social events. Its catalogue includes Christmas crackers.

The firm's founder grins, nearly sheepishly at the joke. But the pun has made the cut and will feature in future crackers.

"The success is gauged by the gag by the number of groans and the loudness of the groans around the table," the founder explains.

The key to a great Christmas cracker joke is not the same as a stand-up joke in itself. It is all about the setting - in this case, the shared amusement of the holiday dinner table with elders, children and possibly friends.

"The goal is for the joke to be a thing that brings the child together with the 80-year-old," she adds.

The Neuroscience Behind Communal Amusement

Coming together to enjoy shared laughter is not only nothing new, scientists argue, it is probably to be pre-human.

"So when you are laughing with people around the Christmas table you are engaging in what's very likely a really primordial mammalian social vocalisation," explains a neuroscience expert.

Communal laughter, she says, helps forge and strengthen social connections between individuals.

Scientists have found that a lack of such social exchanges can significantly harm mental and physical well-being.

"The people you converse with, and laugh with, it results in enhanced levels of 'happy chemical' uptake," the professor continues.

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

"You're not just chuckling at a silly pun with a holiday cracker," she says. "You are in fact performing a lot of the really vital work of building, preserving the connections you have with those you love."

Which Occurs Inside the Mind?

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

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

Using functional magnetic resonance imaging (fMRI), a kind of neural imager which shows which parts of the mind are working harder, researchers have been able to map the regions that get more blood.

Testing involves imaging the brains of volunteer subjects and then exposing them to a database of humorous phrases, accompanied by either a non-emotional sound, or pre-recorded laughter.

"In the scanner we got a really fascinating activation pattern of activation," notes the neuroscientist.

A joke stimulates not just the areas of the mind responsible for hearing and interpreting speech, but also neural regions involved in both preparation and initiating movement and those involved in vision and memory.

Put these elements as a whole, and individuals hearing a pun have a complex series of brain responses that underpin the amusement we hear.

The Infectious Power of Laughter

Researchers discovered that when a humorous word is paired with chuckles there is a stronger response in the mind than the same phrase when followed by a neutral sound.

"This activation occurred in parts of the mind that you would use to move your face into a grin or a chuckle," the professor says.

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

Amusement, according to the professor, can be contagious.

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

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

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

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

The Quest for the Perfect Cracker Joke

Will we ever find the perfect gag?

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

Years ago, a professor established a scientific project for the planet's funniest joke.

More than tens of thousands of jokes submitted, with ratings lodged by hundreds of thousands of participants around the world, he has a better understanding than most as to what succeeds and what does not.

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

"But they also be poor jokes, puns that cause us to moan," he continues.

The increasingly "terrible" the joke, he says the more effective.

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

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

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