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

This one-liner is met by groans that echo through a storage facility in the capital.

We're at a humor-evaluation session with a firm that makes products for social events. Its repertoire features festive crackers.

The firm's owner smiles, almost apologetically at the joke. But the pun has been selected and will appear in future crackers.

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

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

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

The Neuroscience Of Shared Laughter

Coming together to enjoy shared laughter is not only nothing new, experts say, it is likely to be pre-human.

"Therefore when you are chuckling with others at the Christmas dinner you are dropping into what's very likely a truly primordial mammalian play vocalisation," says a neuroscience expert.

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

Researchers have found that a lack of such interactions can seriously harm both psychological and bodily well-being.

"Those you converse with, and share laughter with, it leads to enhanced amounts of 'happy chemical' uptake," she continues.

These natural chemicals are the brain's "happy chemicals" and are produced both to alleviate tension and discomfort and in response to enjoyable experiences, such as laughing with loved ones over a particularly awful Christmas cracker joke.

"It's not simply laughing at a silly joke with a Christmas cracker," she states. "You are actually performing a lot of the really important work of making, maintaining the social bonds you have with those you love."

What Happens Inside the Mind?

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

An awful lot occurs in response to comedy, it turns out.

Employing brain scanning technology, a type of brain scanner which shows which areas of the brain are working harder, researchers have been able to map the areas that get more blood.

Testing entails scanning the brains of volunteer participants and then exposing them to a collection of funny phrases, paired with either a neutral sound, or recorded chuckles.

"In the scanner we observed a very interesting pattern of neural activity," notes the neuroscientist.

A gag activates not just the parts of the mind responsible for hearing and understanding language, but also neural regions associated with both planning and starting motion and those linked to vision and recall.

Put these elements as a whole, and people listening to a joke have a complex series of neural responses that underpin the laughter we hear.

The Infectious Power of Chuckles

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

"This activation occurred in areas of the brain that you would employ to move your face into a smile or a laugh," she says.

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

Amusement, according to the expert, can be contagious.

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

"You laugh more when you know others," she says, "and laughter increases more when you like them or care for them."

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

"It's the laughter. The gag is the terrible Christmas cracker joke, and it's just a reason to laugh together."

The Search for the Perfect Cracker Joke

Will we ever find the perfect gag?

Probably not, but that has not stopped experts from trying to.

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

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

The ideal festive cracker pun must be short, he says.

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

The more "terrible" the gag, he says the better.

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

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

"It creates a common moment at the gathering and I think it's lovely."