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

This joke is met by moans that echo through a warehouse in London.

We're at a joke-testing session with a company that produces supplies for gatherings. Its repertoire includes Christmas crackers.

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

"You measure the gag by the volume of groans and the loudness of the groans at the table," she says.

The secret to a good Christmas cracker joke is not the same as a good gag in itself. It is all about the setting - in this case, the communal amusement of the Christmas dinner table with grandparents, kids and possibly neighbours.

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

The Neuroscience Of Communal Laughter

Gathering to enjoy communal amusement is not only nothing new, scientists say, it is likely to be pre-human.

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

Shared laughter, she says, helps forge and strengthen social bonds between individuals.

Researchers have discovered that a lack of such social exchanges can significantly damage mental and physical health.

"Those you talk to, and laugh with, it leads to enhanced amounts of 'happy chemical' uptake," she adds.

These natural chemicals are the brain's "feel-good compounds" and are produced both to reduce stress and pain and in reaction to pleasurable activities, such as laughing with loved ones over a truly terrible festive cracker joke.

"It's not simply laughing at a silly joke with a holiday cracker," the expert states. "You are in fact performing a lot of the really important work of making, maintaining the connections you have with the people you care about."

Which Happens Inside the Mind?

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

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

Employing brain scanning technology, a type of neural imager which shows which areas of the mind are more active, researchers have been able to map the areas that receive more blood.

Testing entails scanning the minds of healthy participants and then exposing them to a collection of funny phrases, accompanied by either a non-emotional sound, or recorded laughter.

"During the study we observed a very interesting pattern of neural activity," says the neuroscientist.

A gag activates not just the parts of the mind responsible for auditory processing and understanding speech, but also neural areas associated with both preparation and initiating motion and those involved in sight and memory.

Put all of this as a whole, and people listening to a pun have a complex series of brain responses that underpin the laughter we hear.

The Contagious Nature of Chuckles

Researchers discovered that when a funny phrase is paired with chuckles there is a greater response in the mind than the same phrase when accompanied by a non-emotional sound.

"This activation occurred in parts of the brain that you would use to contort your face into a smile or a chuckle," she says.

It indicates we are not just reacting to funny jokes, they are reacting to the laughter that accompanies them.

Laughter, according to the professor, can be contagious.

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

"People laugh harder when you are familiar with others," she notes, "and laughter increases further when you like them or care for them."

When it comes to Christmas cracker jokes, she explains, the feel-good effect is more probable to be caused not by the joke in itself, but from the response to it.

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

The Search for the Ideal Cracker Joke

Will we ever discover the ultimate joke?

Likely not, but that has not prevented experts from attempting to.

In 2001, a psychologist set up a scientific search for the planet's funniest joke.

More than tens of thousands of gags later, with ratings provided by hundreds of thousands of participants globally, he has a clearer idea than many as to what succeeds and what fails.

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

"They must also need to be poor jokes, jokes that make us moan," he continues.

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

"The reason is that 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 find them funny.

"That's a common moment at the table and I think it's lovely."