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"What was the price did Father Christmas's sled cost? Nothing, it was on the house."

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

We're at a joke-testing meeting with a firm that produces products for social events. Its repertoire features festive crackers.

The company's founder smiles, almost apologetically at the gag. But the joke has been selected and will appear in upcoming crackers.

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

The secret to a great Christmas cracker joke is not the same as a good joke in itself. It is all about the context - in this case, the shared amusement of the holiday meal with grandparents, kids and possibly neighbours.

"The goal is for the joke to be a thing that brings the child in harmony with the grandparent," she states.

The Neuroscience Behind Communal Laughter

Gathering to experience shared amusement is not only ancient, scientists say, it is probably to be pre-human.

"So when you are laughing with others around the Christmas dinner you are engaging in what's almost certainly a truly primordial mammalian social vocalisation," says a neuroscience expert.

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

Researchers have found that a lack of these social exchanges can seriously damage both psychological and bodily well-being.

"Those you talk to, and laugh with, it results in increased levels of endorphin uptake," she continues.

Endorphins are the brain's "happy chemicals" and are produced both to reduce stress and pain and in response to enjoyable experiences, such as chuckling with loved ones over a particularly terrible Christmas cracker joke.

"It's not simply chuckling at a silly pun with a Christmas cracker," she states. "You are actually performing a lot of the really important task of making, maintaining the connections you have with the people you care about."

What Happens Inside the Brain?

But what is actually taking place within the mind when we listen to a joke?

An awful lot occurs in response to comedy, it transpires.

Employing brain scanning technology, a kind of neural imager which shows which parts of the mind are working harder, researchers have been able to chart the areas that receive more blood flow.

The research involves imaging the minds of healthy subjects and then subjecting them to a collection of humorous phrases, paired with either a neutral sound, or pre-recorded laughter.

"In the scanner we got a really fascinating pattern of activation," says the neuroscientist.

A gag activates not just the parts of the brain in charge of hearing and understanding language, but also brain regions involved in both planning and starting motion and those involved in sight and recall.

Put all of this together, and individuals hearing a joke have a complex series of brain responses that underpin the amusement we experience.

The Contagious Power of Laughter

Researchers discovered that when a humorous word is paired with chuckles there is a stronger response in the brain than the same word when accompanied by a neutral sound.

"This activation occurred in areas of the brain that you would employ to contort your face into a grin or a laugh," she explains.

It means we are not just reacting to funny words, they are responding to the amusement that follows them.

Laughter, says the professor, can be infectious.

So what does this mean for the laughter found around a holiday gathering?

"People laugh more 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 feel-good effect is more probable to be triggered not by the joke itself, but from the reaction to it.

"It's the laughter. The gag is the dreadful Christmas cracker pun, and it's just a pretext to chuckle as a group."

The Quest for the Perfect Cracker Joke

Is it possible to discover the perfect joke?

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

In 2001, a professor established a research search for the world's most humorous gag.

Over tens of thousands of gags later, with ratings lodged by 350,000 people globally, he has a better understanding than most as to what succeeds and what does not.

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

"They must also be bad gags, jokes that make us moan," he adds.

The increasingly "awful" the gag, he states the better.

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

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

"It creates a shared moment around the table and I think it's lovely."