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

This joke is greeted with groans that echo through a storage facility in London.

This describes a joke-testing meeting with a company that produces supplies for gatherings. Its repertoire includes festive crackers.

The company's owner grins, almost apologetically at the gag. But the joke has been selected and will feature in upcoming crackers.

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

The key to a good Christmas cracker pun is not the same as a stand-up joke in itself. It is all about the context - in this case, the shared laughter of the holiday dinner table with grandparents, kids and possibly neighbours.

"You want the gag to be a thing that brings the child in harmony with the grandparent," she adds.

The Neuroscience Of Communal Amusement

Gathering to enjoy communal laughter is not only ancient, scientists argue, it is probably to be older than humanity.

"So when you are chuckling with people at the Christmas table you are engaging in what's almost certainly a truly primordial mammal play vocalisation," explains a neuroscience expert.

Shared laughter, she says, aids in make and maintain social bonds between individuals.

Scientists have found that a lack of these social exchanges can seriously damage both psychological and bodily health.

"Those you talk to, and laugh with, it leads to enhanced amounts of endorphin release," the professor adds.

These natural chemicals are the brain's "happy chemicals" and are released both to alleviate tension and discomfort and in reaction to enjoyable experiences, such as chuckling with loved ones over a particularly terrible festive cracker joke.

"You're not just laughing at a silly joke with a Christmas cracker," the expert states. "You are in fact performing a lot of the truly vital work of building, preserving the social bonds you have with the people you love."

What Occurs Inside the Brain?

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

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

Using brain scanning technology, a type of neural imager which indicates which areas of the brain are more active, scientists have been able to chart the regions that receive more blood flow.

Testing entails scanning the minds of volunteer participants and then exposing them to a database of humorous words, accompanied by either a neutral sound, or pre-recorded chuckles.

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

A gag activates not just the parts of the brain in charge of auditory processing and interpreting speech, but also neural areas associated with both planning and starting motion and those linked to vision and memory.

Put all of this as a whole, and people hearing a joke have a complex series of brain reactions that support the amusement we experience.

The Infectious Power of Laughter

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

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

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

Amusement, says the professor, can be infectious.

So what does this imply for the laughter found at a Christmas gathering?

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

When it comes to festive cracker puns, she says, the positive effect is more probable to be triggered not by the joke in itself, but from the reaction to it.

"The laughter is key. The gag is the dreadful holiday cracker pun, and it's just a pretext to laugh together."

The Quest for the Perfect Festive Pun

Is it possible to find the perfect gag?

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

In 2001, a professor set up a research search for the planet's funniest joke.

More than tens of thousands of jokes submitted, with scores provided by hundreds of thousands of participants globally, he has a clearer idea than most as to what succeeds and what does not.

The perfect festive cracker pun must be brief, he explains.

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

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

"This is because if nobody finds it funny – it's the gag's fault, not yours.

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

"It creates a shared experience around the gathering and I think it's lovely."