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

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

This describes a joke-testing session with a company that produces supplies for social events. Its repertoire includes Christmas crackers.

The firm's founder grins, almost apologetically at the gag. But the joke has made the cut and will feature in future crackers.

"The success is gauged by the gag by the volume of moans and the loudness of the groans around the table," the founder says.

The secret to a great Christmas cracker pun is not the same as a good joke per se. It is entirely about the context - in this case, the shared laughter of the holiday meal with elders, children and potentially friends.

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

The Science Behind Communal Amusement

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

"So when you are laughing with people around the holiday dinner you are dropping into what's very likely a really ancient mammal play vocalisation," says a neuroscience expert.

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

Researchers have discovered that a absence of such social exchanges can significantly damage mental and physical health.

"The people you talk to, and share laughter with, it results in enhanced amounts of 'happy chemical' release," the professor continues.

These natural chemicals are the brain's "happy chemicals" and are produced both to alleviate stress and pain and in response to enjoyable activities, such as laughing with loved ones over a truly awful festive cracker gag.

"It's not simply laughing at a silly pun with a holiday cracker," the expert says. "You are actually performing a lot of the really vital task of building, preserving the connections you have with those you love."

What Happens In the Mind?

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

A tremendous amount occurs in response to humour, it transpires.

Using functional magnetic resonance imaging (fMRI), a type of neural imager which indicates which areas of the mind are more active, researchers have been able to map the regions that get more blood flow.

The research involves imaging the brains of healthy subjects and then subjecting them to a collection of humorous phrases, accompanied by either a non-emotional sound, or recorded laughter.

"In the scanner we observed a really fascinating activation pattern of neural activity," notes the neuroscientist.

A joke activates not just the parts of the mind in charge of auditory processing and understanding speech, but also neural areas associated with both preparation and initiating motion and those linked to sight and memory.

Combine all of this as a whole, and individuals listening to a joke have a sophisticated set of brain responses that underpin the laughter we experience.

The Contagious Power of Laughter

Researchers found that when a humorous phrase is combined with laughter there is a greater reaction in the mind than the same word when followed by a neutral sound.

"This was in parts of the mind that you would employ to contort your face into a smile or a chuckle," she says.

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

Laughter, according to the expert, can be contagious.

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

"People laugh more when you are familiar with people," she notes, "and you laugh further when you like them or love them."

When it comes to Christmas cracker puns, she explains, the feel-good factor is more likely to be triggered not by the joke itself, but from the response to it.

"The laughter is key. The gag is the terrible holiday cracker pun, and it's just a reason to chuckle as a group."

The Search for the Perfect Cracker Joke

Will we ever discover the ultimate joke?

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

Years ago, a psychologist set up a research search for the planet's most humorous gag.

More than 40,000 jokes submitted, with scores provided by 350,000 people around the world, he has a better idea than many as to what works and what fails.

The ideal festive cracker joke must be short, he explains.

"But they also be bad gags, puns that cause us to moan," he continues.

The increasingly "terrible" the joke, he says the more effective.

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

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

"That's a shared experience at the gathering and I think it's wonderful."