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

This joke is met by groans that echo through a warehouse in London.

We're at a humor-evaluation session with a company that makes products for gatherings. Its repertoire features festive crackers.

The company's founder grins, almost sheepishly at the joke. But the joke has made the cut and will appear in future crackers.

"The success is gauged by the gag by the volume of moans and the intensity of the groans around the table," she says.

The secret to a good Christmas cracker pun is not the identical as a good joke per se. It is entirely about the setting - in this instance, the communal amusement of the Christmas dinner table with grandparents, kids and potentially friends.

"The goal is for the gag to be something that unites the eight-year-old in harmony with the grandparent," she adds.

The Neuroscience Of Shared Laughter

Coming together to enjoy shared amusement is not only ancient, scientists argue, it is likely to be older than humanity.

"So when you are chuckling with others at the Christmas table you are engaging in what's almost certainly a truly ancient mammalian play sound," says a professor.

Communal laughter, she says, helps forge and strengthen social bonds between people.

Scientists have discovered that a lack of these social exchanges can seriously damage mental and physical well-being.

"The people you talk to, and laugh with, it results in enhanced amounts of endorphin release," the professor continues.

These natural chemicals are the body's "feel-good compounds" and are released both to alleviate tension and discomfort and in reaction to enjoyable activities, such as chuckling with friends over a truly terrible festive cracker gag.

"You're not just chuckling at a silly joke with a holiday cracker," the expert states. "You are in fact doing a lot of the really important work of building, preserving the connections you have with the people you care about."

What Happens Inside the Brain?

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

An awful lot happens in response to humour, it transpires.

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

Testing involves scanning the minds of healthy subjects and then exposing them to a collection of funny words, accompanied by either a neutral sound, or pre-recorded chuckles.

"In the scanner we observed a really fascinating activation pattern of activation," says the professor.

A gag activates not just the areas of the mind responsible for auditory processing and understanding language, but also brain areas involved in both planning and initiating motion and those involved in vision and memory.

Put all of this together, and individuals hearing a pun have a sophisticated set of neural responses that underpin the laughter we hear.

The Contagious Power of Chuckles

Scientists found that when a humorous phrase is paired with laughter there is a greater response in the brain than the identical word when accompanied by a non-emotional sound.

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

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

Amusement, says the expert, can be contagious.

So what does this imply for the laughter found around a Christmas gathering?

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

When it comes to Christmas cracker puns, she explains, the positive effect is more likely to be triggered not by the gag itself, but from the reaction to it.

"It's the laughter. The joke is the terrible Christmas cracker pun, and it's just a reason to laugh as a group."

The Quest for the Perfect Festive Pun

Is it possible to find the ultimate joke?

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

Years ago, a psychologist set up a research project for the planet's most humorous joke.

Over tens of thousands of gags later, with scores provided by hundreds of thousands of participants globally, he has a better understanding than most as to what succeeds and what fails.

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

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

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

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

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

"It creates a shared experience at the table and I think it's lovely."