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

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

We're at a humor-evaluation meeting with a firm that makes products for gatherings. Its repertoire includes Christmas crackers.

The firm's owner smiles, almost sheepishly at the gag. But the joke has made the cut and will appear in future crackers.

"You measure the gag by the volume of groans and the loudness of the groans around the table," she explains.

The key to a good Christmas cracker joke is not the same as a stand-up gag in itself. It is entirely about the context - in this case, the shared amusement of the Christmas meal with grandparents, kids and potentially friends.

"The goal is for the gag to be a thing that brings the child in harmony with the grandparent," she adds.

The Science Behind Communal Laughter

Gathering to experience shared laughter is not only nothing new, experts say, it is probably to be older than humanity.

"So when you are laughing with people at the holiday dinner you are engaging in what's very likely a really primordial mammal social vocalisation," explains a professor.

Shared laughter, she says, aids in forge and strengthen social connections between individuals.

Researchers have found that a absence of such social exchanges can seriously harm mental and physical well-being.

"Those you talk to, and laugh with, it results in enhanced amounts of 'happy chemical' release," she adds.

These natural chemicals are the brain's "happy chemicals" and are released both to alleviate tension and discomfort and in response to pleasurable activities, such as laughing with loved ones over a truly terrible festive cracker gag.

"You're not just laughing at a foolish joke with a Christmas cracker," she states. "You are actually performing a lot of the really vital work of making, maintaining the social bonds you have with those you love."

Which Happens In the Mind?

But what is actually taking place within the brain when we listen to a joke?

A tremendous amount occurs in reaction to humour, it turns out.

Using brain scanning technology, a type of neural imager which indicates which parts of the mind are working harder, scientists have been able to map the regions that receive more blood.

The research entails scanning the brains of healthy subjects and then subjecting them to a database of funny phrases, paired with either a non-emotional sound, or recorded chuckles.

"In the scanner we got a very interesting pattern of activation," says the professor.

A joke activates not just the areas of the brain in charge of auditory processing and understanding speech, but also brain areas involved in both planning and starting movement and those linked to vision and recall.

Put all of this as a whole, and people hearing a joke have a sophisticated series of brain responses that underpin the laughter we hear.

The Contagious Nature of Chuckles

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

"This activation occurred in parts of the brain that you would employ to move your face into a smile or a laugh," she says.

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

Amusement, says the professor, can be infectious.

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

"You laugh more when you know others," she says, "and laughter increases more when you like them or care for them."

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

"The laughter is key. The joke is the dreadful holiday cracker joke, and it's just a pretext to chuckle as a group."

The Quest for the Ideal Cracker Joke

Is it possible to find the ultimate gag?

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

Years ago, a professor established a scientific search for the planet's funniest gag.

Over tens of thousands of gags submitted, with ratings lodged by hundreds of thousands of participants around the world, he has a better idea than most as to what works and what does not.

The ideal festive cracker joke needs to be short, he explains.

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

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

"This is because if no-one finds it funny – it's the joke's fault, not yours.

"What's interesting about the holiday cracker jokes is that none of us find them humorous.

"It creates a common moment around the gathering and I think it's lovely."