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"How much did Santa's sled cost? Zero, it was on the house."

This one-liner is greeted with groans that echo through a warehouse in London.

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

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

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

The secret to a great holiday cracker joke is not the same as a good joke per se. It is all about the setting - in this case, the shared amusement of the holiday meal with grandparents, children and potentially friends.

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

The Science Of Communal Laughter

Coming together to experience communal amusement is not only nothing new, scientists argue, it is probably to be older than humanity.

"So when you are laughing with others around the Christmas dinner you are engaging in what's almost certainly a truly primordial mammalian social sound," says a neuroscience expert.

Shared amusement, she says, aids in make and maintain social connections between individuals.

Researchers have discovered that a lack of such interactions can seriously harm mental and physical well-being.

"Those you converse with, and share laughter with, it leads to enhanced levels of endorphin release," she adds.

These natural chemicals are the brain's "happy chemicals" and are released both to reduce stress and pain and in response to pleasurable activities, such as chuckling with loved ones over a truly terrible Christmas cracker joke.

"It's not simply laughing at a silly pun with a Christmas cracker," she states. "You are in fact doing a lot of the truly important work of building, preserving the social bonds you have with those you care about."

What Occurs In the Brain?

But what is actually happening inside the mind when we hear a gag?

An awful lot occurs in reaction to humour, it transpires.

Using functional magnetic resonance imaging (fMRI), a kind of brain scanner which shows which parts of the brain are more active, researchers have been able to chart the areas that receive more blood flow.

The research entails imaging the brains of volunteer subjects and then subjecting them to a collection of humorous phrases, paired with either a non-emotional sound, or recorded chuckles.

"In the scanner we observed a really interesting activation pattern of neural activity," says the professor.

A gag stimulates not just the parts of the mind in charge of hearing and understanding speech, but also brain areas associated with both preparation and starting motion and those involved in vision and memory.

Put all of this as a whole, and individuals listening to a pun have a complex set of neural reactions that underpin the amusement we hear.

The Infectious Power of Chuckles

Scientists discovered that when a humorous word is combined with laughter there is a stronger reaction in the brain than the same phrase when accompanied by a non-emotional sound.

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

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

Laughter, says the professor, can be contagious.

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

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

When it comes to Christmas cracker jokes, she explains, the feel-good factor is more likely to be caused not by the gag itself, but from the reaction to it.

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

The Quest for the Perfect Cracker Joke

Will we ever discover the perfect gag?

Probably not, but that has not stopped researchers from attempting to.

In 2001, a psychologist set up a scientific search for the planet's funniest gag.

More than 40,000 gags submitted, with ratings lodged by 350,000 people around the world, he has a better idea than most as to what succeeds and what does not.

The ideal festive cracker pun must be short, he says.

"But they also need to be bad gags, puns that cause us to moan," he continues.

The increasingly "awful" the joke, he says the better.

"The reason is that if no-one laughs – it's the joke's fault, 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 lovely."