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

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

We're at a humor-evaluation meeting with a firm that produces supplies for gatherings. Its repertoire features festive crackers.

The firm's owner grins, nearly apologetically at the gag. But the pun has made the cut and will feature in future crackers.

"The success is gauged by the joke by the number of groans and the loudness of the groans at the table," the founder explains.

The secret to a great holiday cracker joke is not the identical as a stand-up joke in itself. It is entirely about the setting - in this instance, the shared amusement of the holiday meal with elders, children and potentially friends.

"The goal is for the gag to be something that brings the eight-year-old in harmony with the 80-year-old," she adds.

The Science Behind Shared Amusement

Coming together to enjoy communal laughter is not only nothing new, experts say, it is probably to be older than humanity.

"So when you are laughing with others at the holiday dinner you are dropping into what's almost certainly a really primordial mammalian play vocalisation," explains a neuroscience expert.

Communal amusement, she says, helps make and maintain social connections between individuals.

Researchers have found that a lack of these interactions can significantly damage mental and physical health.

"Those you converse with, and share laughter with, it leads to enhanced amounts of endorphin release," she continues.

Endorphins are the brain's "feel-good compounds" and are released both to alleviate tension and discomfort and in reaction to enjoyable experiences, such as chuckling with loved ones over a particularly awful festive cracker joke.

"You're not just laughing at a silly pun with a Christmas cracker," the expert states. "You are in fact performing a lot of the really important task of building, preserving the connections you have with those you care about."

What Happens Inside the Brain?

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

A tremendous amount happens in response to humour, it turns out.

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

The research involves scanning the minds of healthy participants and then subjecting them to a collection of humorous words, accompanied by either a neutral sound, or pre-recorded laughter.

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

A joke activates not just the areas of the brain in charge of hearing and understanding speech, but also brain areas involved in both preparation and starting movement and those linked to vision and memory.

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

The Contagious Nature of Laughter

Scientists discovered that when a humorous phrase is paired with chuckles there is a stronger reaction in the brain than the same word when accompanied by a non-emotional sound.

"This activation occurred in areas of the brain that you would employ to move your expression into a smile or a chuckle," the professor explains.

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

Laughter, says the expert, can be contagious.

So what does this mean for the chuckles heard around a Christmas gathering?

"People laugh harder when you know people," she notes, "and you laugh further when you like them or care for them."

When it comes to festive cracker puns, she explains, the positive factor is more probable to be caused not by the joke in itself, but from the reaction to it.

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

The Quest for the Ideal Cracker Joke

Will we ever find the perfect joke?

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

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

More than tens of thousands of jokes submitted, with scores lodged by hundreds of thousands of participants globally, he has a clearer understanding than most as to what succeeds and what fails.

The ideal Christmas cracker joke needs to be short, he says.

"They must also need to be bad gags, puns that cause us to groan," he continues.

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

"The reason is that if no-one finds it funny – it's the gag's shortcoming, not yours.

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

"It creates a common moment at the table and I believe it's lovely."