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

This one-liner is met by groans that echo through a warehouse in London.

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

The company's founder grins, almost apologetically at the gag. But the pun has been selected and will feature in upcoming crackers.

"You measure the gag by the volume of groans and the intensity of the groans at the table," she says.

The key to a good Christmas cracker pun is not the same as a good joke per se. It is entirely about the context - in this case, the communal amusement of the holiday dinner table with grandparents, kids and possibly neighbours.

"The goal is for the joke to be something that brings the child together with the 80-year-old," she states.

The Science Behind Communal Amusement

Gathering to enjoy communal amusement is not only nothing new, experts say, it is probably to be pre-human.

"So when you are laughing with others around the holiday table you are engaging in what's almost certainly a really ancient mammal social sound," explains a neuroscience expert.

Shared amusement, she explains, helps forge and strengthen social connections between individuals.

Researchers have found that a lack of such interactions can significantly damage both psychological and bodily well-being.

"Those you talk to, and laugh with, it results in increased levels of 'happy chemical' uptake," she continues.

Endorphins are the body's "feel-good compounds" and are released both to alleviate tension and discomfort and in response to pleasurable experiences, such as laughing with friends over a truly awful festive cracker gag.

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

Which Occurs Inside the Mind?

But what is truly taking place within the mind when we listen to a joke?

An awful lot happens in response to comedy, it turns out.

Employing functional magnetic resonance imaging (fMRI), a kind of neural imager which shows which parts of the mind are more active, scientists have been able to chart the regions that receive more blood.

Testing involves imaging the brains of healthy participants and then exposing them to a collection of humorous words, accompanied by either a non-emotional sound, or pre-recorded laughter.

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

A gag stimulates not just the parts of the mind responsible for hearing and understanding speech, but also neural areas associated with both planning and starting motion and those linked to sight and memory.

Combine these elements as a whole, and people hearing a joke have a sophisticated set of neural responses that underpin the laughter we hear.

The Infectious Power of Laughter

Researchers discovered that when a funny word is combined with laughter there is a greater response in the brain than the identical phrase when accompanied by a neutral sound.

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

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

Amusement, according to the expert, can be contagious.

So what does this imply for the chuckles found around a holiday gathering?

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

When it comes to festive cracker jokes, she says, the positive factor is more likely to be triggered not by the joke itself, but from the response to it.

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

The Search for the Ideal Festive Pun

Is it possible to find the perfect gag?

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

In 2001, a professor established a research project for the planet's funniest joke.

More than 40,000 jokes submitted, with ratings provided by 350,000 participants around the world, he has a clearer understanding than many as to what works and what fails.

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

"But they also be poor jokes, jokes that cause us to groan," he continues.

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

"The reason is that if no-one laughs – it's the gag's fault, not yours.

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

"It creates a common experience around the gathering and I think it's wonderful."