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

This one-liner is greeted with groans that resonate through a storage facility in London.

We're at a joke-testing meeting with a company that makes supplies for social events. Its catalogue includes festive crackers.

The company's owner smiles, nearly apologetically at the joke. But the joke has made the cut and will appear in upcoming crackers.

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

The key to a good holiday cracker joke is not the same as a stand-up joke per se. It is all about the context - in this instance, the communal laughter of the Christmas dinner table with elders, children and possibly neighbours.

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

The Science Of Shared Amusement

Coming together to enjoy shared laughter is not only ancient, experts say, it is likely to be pre-human.

"So when you are laughing with people around the Christmas dinner you are engaging in what's almost certainly a truly ancient mammal play sound," explains a professor.

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

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

"The people you converse with, and share laughter with, it results in increased levels of endorphin uptake," she continues.

These natural chemicals are the brain's "happy chemicals" and are produced both to alleviate stress and pain and in response to pleasurable experiences, such as laughing with friends over a particularly terrible festive cracker joke.

"You're not just laughing at a foolish joke with a holiday cracker," the expert says. "You are actually doing a lot of the really vital work of making, maintaining the social bonds you have with the people you care about."

Which Happens Inside the Brain?

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

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

Employing brain scanning technology, a kind of brain scanner which shows which parts of the mind are more active, researchers have been able to map the areas that receive more blood.

Testing entails scanning the brains of healthy subjects and then subjecting them to a collection of funny words, paired with either a neutral sound, or pre-recorded laughter.

"In the scanner we got a very fascinating activation pattern of neural activity," says the professor.

A gag stimulates not just the areas of the brain in charge of auditory processing and understanding language, but also neural regions involved in both planning and initiating motion and those involved in vision and memory.

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

The Contagious Nature of Chuckles

Scientists discovered that when a humorous word is paired with laughter there is a greater reaction in the mind than the identical phrase when followed by a neutral sound.

"This was in areas of the brain that you would employ to contort your face into a grin or a laugh," the professor says.

It means people are not just responding to humorous words, they are responding to the laughter that follows them.

Laughter, says the professor, can be infectious.

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

"You laugh more when you know others," she notes, "and you laugh 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 triggered not by the gag itself, but from the response to it.

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

The Quest for the Ideal Festive Pun

Is it possible to find the perfect joke?

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

In 2001, a psychologist established a scientific search for the planet's most humorous gag.

More than tens of thousands of jokes later, with ratings provided by 350,000 participants around the world, he has a better understanding than most as to what works and what fails.

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

"They must also need to be bad jokes, jokes that make us groan," he adds.

The more "awful" the joke, he states the better.

"This is because if nobody laughs – it's the joke's shortcoming, not yours.

"The fascinating part about the Christmas cracker puns is that not one person find them funny.

"That's a common moment around the table and I believe it's wonderful."