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

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

We're at a humor-evaluation session with a company that produces supplies for social events. Its repertoire includes Christmas crackers.

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

"The success is gauged by the joke by the number of groans and the loudness of the groans at the table," she says.

The key to a good holiday cracker joke is not the same as a stand-up joke in itself. It is entirely about the context - in this case, the shared amusement of the holiday meal with grandparents, children and potentially neighbours.

"You want the gag to be a thing that brings the child together with the grandparent," she states.

The Neuroscience Of Communal Amusement

Coming together to experience communal laughter is not only ancient, experts argue, it is likely to be older than humanity.

"So when you are laughing with people at the Christmas dinner you are dropping into what's very likely a truly primordial mammal play vocalisation," says a neuroscience expert.

Shared laughter, she says, helps make and maintain social bonds between people.

Scientists have discovered that a absence of these social exchanges can seriously harm both psychological and bodily health.

"Those you converse with, and laugh with, it results in increased levels of endorphin release," the professor adds.

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

"You're not just chuckling at a foolish pun with a Christmas cracker," she states. "You are in fact performing a lot of the really important task of making, maintaining the social bonds you have with those you care about."

What Happens Inside the Brain?

But what is actually taking place inside the brain when we hear a gag?

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

Employing brain scanning technology, a type of brain scanner which indicates which parts of the mind are working harder, researchers have been able to map the areas that receive more blood.

Testing involves scanning the brains of healthy participants and then exposing them to a collection of funny words, paired with either a neutral sound, or pre-recorded laughter.

"During the study we observed a really fascinating activation pattern of activation," says the neuroscientist.

A joke activates not just the areas of the mind in charge of hearing and understanding language, but also neural areas associated with both planning and starting movement and those involved in vision and recall.

Combine all of this as a whole, and individuals listening to a pun have a sophisticated series of brain reactions that underpin the laughter we hear.

The Infectious Power of Laughter

Researchers discovered that when a funny phrase is paired with chuckles there is a greater reaction in the brain than the same phrase when followed by a non-emotional sound.

"This was in parts of the mind that you would employ to contort your face into a grin or a laugh," she says.

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

Amusement, according to the expert, can be contagious.

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

"You laugh harder when you know others," she notes, "and you laugh further when you like them or love them."

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

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

The Search for the Ideal Festive Pun

Is it possible to discover the perfect joke?

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

Years ago, a psychologist established a research search for the planet's funniest joke.

More than tens of thousands of gags later, with ratings lodged by 350,000 people globally, he has a better idea than many as to what works and what fails.

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

"But they also need to be poor gags, jokes that cause us to moan," he adds.

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

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

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

"That's a shared experience around the gathering and I believe it's lovely."