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

This quip is met by groans that resonate through a warehouse in London.

This describes a humor-evaluation session with a company that makes supplies for gatherings. Its repertoire includes festive crackers.

The firm's founder grins, almost 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 around the table," she explains.

The key to a good Christmas cracker pun is not the same as a stand-up joke in itself. It is entirely about the context - in this case, the communal amusement of the Christmas dinner table with elders, children and possibly neighbours.

"The goal is for the gag to be a thing that brings the child in harmony with the 80-year-old," she states.

The Science Behind Shared Laughter

Gathering to enjoy shared laughter is not only ancient, experts say, it is probably to be pre-human.

"So when you are laughing with people at the holiday table you are engaging in what's very likely a truly primordial mammalian play sound," says a professor.

Communal amusement, she says, helps make and maintain social connections between individuals.

Researchers have found that a absence of these interactions can significantly harm both psychological and bodily health.

"The people you converse with, and share laughter with, it leads to enhanced levels of 'happy chemical' uptake," she continues.

Endorphins are the body's "feel-good compounds" and are produced both to alleviate stress and pain and in response to pleasurable activities, such as chuckling with friends over a particularly awful festive cracker joke.

"You're not just chuckling at a foolish joke with a holiday cracker," the expert says. "You are actually doing a lot of the really vital task of building, preserving the social bonds you have with the people you care about."

What Happens In the Mind?

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

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

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

Testing entails imaging the brains of healthy participants and then subjecting them to a database of funny phrases, paired with either a neutral sound, or recorded laughter.

"In the scanner we observed a very fascinating activation pattern of activation," says the neuroscientist.

A gag activates not just the areas of the mind responsible for hearing and understanding speech, but also brain regions associated with both planning and initiating movement and those linked to sight and recall.

Put all of this together, and individuals hearing a pun have a complex series of neural reactions that underpin the laughter we hear.

The Contagious Nature of Laughter

Researchers found that when a humorous word is combined with chuckles there is a greater reaction in the brain than the same word when followed by a non-emotional sound.

"This was in parts of the brain that you would employ to contort your face into a smile or a chuckle," the professor explains.

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

Laughter, says the professor, can be contagious.

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

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

When it comes to Christmas cracker jokes, she explains, the feel-good effect is more probable to be caused not by the gag in itself, but from the response to it.

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

The Quest for the Perfect Cracker Joke

Will we ever discover the perfect gag?

Likely not, but that has not prevented researchers from attempting to.

Years ago, a professor set up a research search for the world's funniest joke.

Over tens of thousands of jokes submitted, with scores provided by 350,000 participants around the world, he has a clearer idea than most as to what succeeds and what fails.

The perfect festive cracker joke needs to be short, he says.

"They must also be poor jokes, jokes that make us moan," he continues.

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

"The reason is that if nobody finds it funny – it's the joke's fault, not yours.

"The fascinating part about the holiday cracker puns is that none of us considers them funny.

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