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

This one-liner is met by moans that echo through a storage facility in London.

This describes a joke-testing session with a firm that produces supplies for gatherings. Its repertoire features festive crackers.

The company's founder grins, nearly sheepishly at the gag. But the pun has made the cut and will appear in upcoming crackers.

"You measure the joke by the number of groans and the intensity of the groans around the table," she explains.

The secret to a good holiday cracker joke is not the same as a stand-up joke per se. It is entirely about the setting - in this case, the communal amusement of the holiday dinner table with elders, children and potentially neighbours.

"You want the joke to be a thing that unites the child in harmony with the grandparent," she states.

The Science Behind Shared Laughter

Coming together to experience communal amusement is not only nothing new, scientists argue, it is probably to be pre-human.

"So when you are laughing with people at the Christmas dinner you are engaging in what's almost certainly a truly primordial mammal social vocalisation," says a professor.

Communal amusement, she says, aids in make and maintain social bonds between individuals.

Scientists have found that a lack of these interactions can significantly harm mental and physical health.

"The people you converse with, and laugh with, it leads to enhanced levels of endorphin release," she continues.

Endorphins are the body's "happy chemicals" and are produced both to reduce stress and pain and in response to pleasurable activities, such as laughing with loved ones over a particularly terrible Christmas cracker joke.

"You're not just laughing at a foolish joke with a holiday cracker," the expert says. "You are in fact doing a lot of the really vital task of making, maintaining the connections you have with those you love."

Which Occurs In the Mind?

But what is actually happening within the mind when we hear a gag?

An awful lot happens in reaction to humour, it turns out.

Using brain scanning technology, a kind of neural imager which indicates which areas of the brain are working harder, researchers have been able to map the areas that receive more blood.

Testing involves imaging the brains of healthy subjects and then exposing them to a database of funny phrases, accompanied by either a neutral sound, or recorded chuckles.

"During the study we got a really fascinating activation pattern of activation," says the professor.

A gag activates not just the parts of the mind in charge of auditory processing and interpreting language, but also brain regions involved in both preparation and initiating motion and those linked to sight and memory.

Put all of this as a whole, and people hearing a joke have a complex series of neural responses that underpin the laughter we experience.

The Infectious Nature of Laughter

Researchers discovered that when a humorous phrase is combined with chuckles there is a greater reaction in the brain than the same word when followed by a non-emotional sound.

"This was in parts of the brain that you would employ to contort your face into a smile or a laugh," the professor explains.

It indicates people are not just reacting to humorous 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 at a holiday gathering?

"You laugh harder when you are familiar with people," she notes, "and you laugh further when you are fond of them or love them."

When it comes to festive cracker jokes, she says, the positive effect is more likely to be caused not by the gag in itself, but from the response to it.

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

The Quest for the Perfect Cracker Joke

Will we ever discover the ultimate joke?

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

Years ago, a professor established a research project for the planet's funniest joke.

More than tens of thousands of jokes later, with ratings lodged by 350,000 people globally, he has a clearer idea than many as to what works and what fails.

The ideal festive cracker pun needs to be short, he says.

"But they also be bad jokes, jokes that cause us to moan," he adds.

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

"This is because if nobody finds it funny – it's the joke's fault, not yours.

"What's interesting about the Christmas cracker puns is that not one person considers them funny.

"It creates a common moment at the gathering and I think it's wonderful."