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

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

We're at a joke-testing session with a company that produces supplies for social events. Its catalogue features Christmas crackers.

The company's owner grins, nearly apologetically at the gag. But the joke has made the cut and will feature in upcoming crackers.

"The success is gauged by the gag by the volume of moans and the loudness of the groans at the table," she explains.

The key to a good Christmas cracker joke is not the same as a good joke in itself. It is all about the setting - in this case, the communal laughter of the holiday meal with elders, kids and possibly neighbours.

"You want the gag to be something that unites the child in harmony with the grandparent," she states.

The Neuroscience Of Communal Laughter

Coming together to enjoy shared laughter is not only ancient, experts argue, it is probably to be older than humanity.

"So when you are chuckling with others at the holiday table you are engaging in what's very likely a really ancient mammalian play sound," explains a neuroscience expert.

Shared laughter, she says, aids in forge and strengthen social bonds between people.

Scientists have found that a lack of such social exchanges can seriously damage mental and physical health.

"The people you talk to, and share laughter with, it results in increased amounts of endorphin release," she adds.

These natural chemicals are the brain's "happy chemicals" and are produced both to reduce stress and pain and in reaction to pleasurable experiences, such as chuckling with loved ones over a particularly terrible festive cracker gag.

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

Which Happens Inside the Mind?

But what is truly taking place within the mind when we hear a gag?

A tremendous amount occurs in reaction to humour, it turns out.

Using brain scanning technology, a type of brain scanner which indicates which parts of the mind are working harder, scientists have been able to map the regions that get more blood.

The research involves imaging the minds of healthy subjects and then subjecting them to a collection of humorous words, accompanied by either a neutral sound, or recorded chuckles.

"In the scanner we observed a really fascinating pattern of neural activity," says the professor.

A gag activates not just the parts of the mind responsible for hearing and interpreting language, but also neural regions associated with both preparation and initiating motion and those involved in sight and recall.

Combine these elements as a whole, and people listening to a joke have a sophisticated series of brain reactions that underpin the laughter we hear.

The Contagious Power of Chuckles

Scientists found that when a funny word is combined with laughter there is a stronger response in the mind than the identical phrase when followed by a non-emotional sound.

"This activation occurred in parts of the mind that you would use to move your face into a smile or a laugh," she says.

It indicates we are not just responding to humorous words, they are responding to the laughter that follows them.

Amusement, says the expert, can be contagious.

So what does this mean for the chuckles found around a holiday table?

"People laugh more when you are familiar with people," she notes, "and you laugh more when you are fond of them or love them."

When it comes to festive cracker jokes, she says, the positive effect is more likely to be caused not by the gag in itself, but from the response to it.

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

The Search for the Perfect Cracker Joke

Is it possible to find the perfect gag?

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

Years ago, a psychologist set up a scientific search for the world's most humorous joke.

More than 40,000 gags later, with ratings provided by 350,000 participants around the world, he has a better understanding than most as to what works and what fails.

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

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

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

"This is because if no-one finds it funny – it's the gag's fault, not yours.

"The fascinating part about the holiday cracker jokes is that none of us considers them humorous.

"That's a shared moment at the gathering and I think it's wonderful."