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"What was the price did Santa's sleigh cost? Zero, it was on the house."

This quip is met by groans that echo through a storage facility in London.

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

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

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

The secret to a good holiday cracker pun is not the same as a stand-up gag in itself. It is entirely about the context - in this instance, the communal amusement of the holiday dinner table with grandparents, children and potentially neighbours.

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

The Neuroscience Behind Shared Laughter

Coming together to experience communal amusement is not only ancient, scientists say, it is likely to be pre-human.

"So when you are laughing with people around the Christmas dinner you are dropping into what's very likely a really ancient mammalian play vocalisation," says a neuroscience expert.

Communal laughter, she explains, helps forge and strengthen social connections between people.

Scientists have found that a absence of these interactions can seriously damage both psychological and bodily well-being.

"The people you converse with, and laugh with, it results in increased amounts of endorphin release," the professor adds.

These natural chemicals are the body's "happy chemicals" and are produced both to alleviate stress and pain and in reaction to enjoyable activities, such as chuckling with loved ones over a truly terrible festive cracker joke.

"It's not simply chuckling at a silly pun with a holiday cracker," the expert says. "You are in fact doing a lot of the really vital task of making, maintaining the social bonds you have with those you love."

Which Happens Inside the Mind?

But what is truly happening within the brain when we hear a joke?

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

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

The research involves imaging the minds of volunteer participants and then subjecting them to a database of funny phrases, accompanied by either a non-emotional sound, or recorded laughter.

"During the study we got a very interesting activation pattern of activation," notes the neuroscientist.

A gag activates not just the parts of the brain responsible for auditory processing and understanding language, but also brain regions involved in both preparation and starting motion and those linked to vision and memory.

Put these elements together, and individuals hearing a pun have a sophisticated set of neural responses that support the amusement we experience.

The Infectious Power of Laughter

Researchers found that when a humorous word is paired with laughter 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 brain that you would use to contort your expression into a grin or a chuckle," she says.

It means we are not just responding to funny jokes, they are responding to the amusement that accompanies them.

Amusement, says the expert, can be contagious.

So what does this mean for the chuckles heard around a Christmas gathering?

"People laugh more when you know people," she notes, "and laughter increases more when you are fond of them or care for them."

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

"It's the laughter. The gag is the dreadful Christmas cracker joke, and it's just a reason to laugh together."

The Search for the Ideal Festive Pun

Is it possible to discover the ultimate joke?

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

Years ago, a professor established a scientific search for the world's funniest joke.

More than 40,000 gags later, with scores provided by hundreds of thousands of people globally, he has a better understanding than many as to what succeeds and what does not.

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

"They must also need to be bad jokes, puns that cause us to moan," he continues.

The increasingly "terrible" the gag, he states the better.

"The reason is that if nobody finds it funny – it's the gag's shortcoming, not your own.

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

"That's a common experience around the table and I believe it's lovely."