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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 resonate through a warehouse in London.

This describes a joke-testing meeting with a company that makes products for social events. Its repertoire features Christmas crackers.

The firm's founder smiles, almost sheepishly at the joke. But the pun has been selected and will feature in upcoming crackers.

"You measure the joke by the volume of groans and the loudness of the groans around the table," the founder explains.

The key to a great holiday cracker joke is not the identical as a stand-up gag per se. It is entirely about the context - in this case, the shared amusement of the Christmas meal with elders, kids and possibly neighbours.

"You want the gag to be something that unites the eight-year-old together with the grandparent," she states.

The Neuroscience Of Shared Amusement

Gathering to enjoy shared laughter is not only nothing new, experts argue, it is probably to be pre-human.

"So when you are chuckling with others around the holiday table you are dropping into what's very likely a really primordial mammal play sound," says a professor.

Shared amusement, she explains, helps make and maintain social bonds between individuals.

Researchers have discovered that a absence of these interactions can seriously damage mental and physical health.

"Those you talk to, and laugh with, it leads to enhanced levels of endorphin uptake," the professor continues.

Endorphins are the brain's "happy chemicals" and are released both to reduce tension and discomfort and in reaction to enjoyable experiences, such as laughing with loved ones over a particularly terrible Christmas cracker joke.

"You're not just laughing at a foolish joke with a Christmas cracker," she says. "You are actually performing a lot of the truly vital task of building, preserving the social bonds you have with the people you care about."

Which Happens Inside the Mind?

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

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

Using brain scanning technology, a type of neural imager which shows which areas of the brain are working harder, scientists have been able to map the regions that get more blood flow.

Testing involves scanning the brains of healthy participants and then exposing them to a collection of humorous phrases, accompanied by either a neutral sound, or pre-recorded chuckles.

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

A gag stimulates not just the areas of the mind in charge of auditory processing and understanding language, but also neural areas involved in both preparation and starting movement and those linked to sight and recall.

Combine these elements as a whole, and people hearing a joke have a complex series of neural responses that support the laughter we hear.

The Contagious Nature of Laughter

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

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

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

Amusement, says the professor, can be infectious.

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

"You laugh more when you are familiar with others," she says, "and laughter increases further when you like them or care for them."

When it comes to Christmas cracker puns, she says, the positive factor is more probable to be triggered not by the gag in itself, but from the response to it.

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

The Search for the Perfect Festive Pun

Will we ever find the perfect joke?

Likely not, but that has not stopped researchers from trying to.

In 2001, a professor set up a scientific project for the world's most humorous gag.

More than tens of thousands of gags submitted, with scores provided by hundreds of thousands of participants globally, he has a clearer idea than most as to what works and what fails.

The perfect Christmas cracker pun needs to be brief, he explains.

"But they also be bad gags, jokes that make us groan," he continues.

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

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

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

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