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

This quip is greeted with groans that resonate through a storage facility in London.

We're at a joke-testing session with a company that makes products for gatherings. Its repertoire includes festive crackers.

The firm's owner grins, nearly apologetically at the joke. But the pun has been selected and will feature in upcoming crackers.

"The success is gauged by the gag by the number of moans and the intensity of the groans at the table," the founder says.

The key to a good holiday cracker pun is not the identical as a stand-up gag per se. It is entirely about the context - in this case, the communal laughter of the Christmas meal with grandparents, kids and possibly friends.

"You want the joke to be a thing that brings the eight-year-old together with the grandparent," she states.

The Neuroscience Of Communal Amusement

Coming together to enjoy communal amusement is not only ancient, scientists argue, it is likely to be pre-human.

"Therefore when you are laughing with others at the holiday table you are dropping into what's almost certainly a truly ancient mammalian social vocalisation," says a professor.

Communal laughter, she explains, helps make and maintain social bonds between people.

Scientists have discovered that a absence of such social exchanges can seriously damage both psychological and bodily well-being.

"Those you talk to, and laugh with, it results in increased levels 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 reaction to enjoyable activities, such as laughing with loved ones over a particularly terrible festive cracker gag.

"You're not just laughing at a foolish joke with a Christmas cracker," she says. "You are in fact performing a lot of the really important task of making, maintaining the social bonds you have with those you care about."

What Happens In the Mind?

But what is actually taking place inside the mind when we hear a joke?

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

Using functional magnetic resonance imaging (fMRI), a type of neural imager which indicates which parts of the brain are more active, scientists have been able to chart the regions that receive more blood flow.

Testing entails imaging the brains of healthy participants and then subjecting them to a collection of humorous words, paired with either a neutral sound, or pre-recorded laughter.

"In the scanner we observed a very interesting pattern of activation," says the neuroscientist.

A gag stimulates not just the areas of the brain in charge of auditory processing and understanding speech, but also brain areas involved in both preparation and initiating motion and those involved in sight and memory.

Combine these elements together, and people hearing a pun have a complex set of brain responses that underpin the amusement we experience.

The Contagious Nature of Laughter

Scientists discovered that when a humorous word is combined with chuckles there is a stronger reaction in the mind than the identical phrase when followed by a non-emotional sound.

"This activation occurred in areas of the brain that you would use to contort your expression into a grin or a laugh," the professor says.

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

Laughter, according to the expert, can be infectious.

So what does this imply for the laughter heard at a holiday gathering?

"People laugh more when you know others," she says, "and you laugh more when you like them or love them."

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

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

The Search for the Perfect Festive Pun

Will we ever find the ultimate joke?

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

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

Over tens of thousands of gags submitted, with scores provided by 350,000 people around the world, he has a better idea than most as to what works and what does not.

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

"They must also be bad jokes, puns that make us moan," he continues.

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

"The reason is that if no-one laughs – it's the gag's shortcoming, not your own.

"What's interesting about the Christmas cracker jokes is that none of us find them humorous.

"That's a common moment around the gathering and I believe it's lovely."