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

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

This describes a joke-testing session with a firm that makes supplies for social events. Its repertoire includes festive crackers.

The firm's founder grins, almost sheepishly at the gag. But the joke has made the cut and will appear in future crackers.

"You measure the gag by the volume of groans and the intensity of the groans around the table," the founder explains.

The secret to a good holiday cracker joke is not the same as a good gag in itself. It is entirely about the context - in this instance, the communal laughter of the holiday dinner table with grandparents, children and possibly neighbours.

"You want the joke to be a thing that unites the child together with the grandparent," she adds.

The Neuroscience Behind Shared Amusement

Coming together to enjoy shared amusement is not only nothing new, scientists say, it is likely to be older than humanity.

"So when you are chuckling with others around the Christmas table you are dropping into what's almost certainly a really primordial mammalian social sound," explains a professor.

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

Scientists have discovered that a lack of such interactions can seriously damage mental and physical health.

"The people you talk to, and share laughter with, it results in increased amounts of 'happy chemical' uptake," she adds.

These natural chemicals are the brain's "happy chemicals" and are released both to alleviate tension and discomfort and in reaction to enjoyable activities, such as laughing with friends over a truly awful Christmas cracker gag.

"It's not simply chuckling at a silly joke with a holiday cracker," the expert states. "You are actually doing a lot of the truly important work of building, preserving the connections you have with the people you love."

What Occurs In the Mind?

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

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

Employing brain scanning technology, a kind of neural imager which indicates which parts of the mind are more active, scientists have been able to chart the areas that receive more blood flow.

Testing entails imaging the brains of healthy participants and then exposing them to a database of funny words, paired with either a neutral sound, or recorded chuckles.

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

A joke stimulates not just the parts of the mind responsible for hearing and understanding speech, but also neural areas associated with both preparation and initiating motion and those linked to vision and memory.

Combine these elements together, and individuals listening to a joke have a complex set of neural reactions that support the laughter we experience.

The Infectious Power of Laughter

Researchers discovered that when a humorous word is combined with laughter there is a greater reaction 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 employ to move your face into a smile or a chuckle," she says.

It means people are not just reacting to humorous words, they are reacting to the laughter that follows them.

Laughter, according to the professor, can be infectious.

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

"You laugh more when you are familiar with others," she notes, "and you laugh further when you are fond of them or love them."

When it comes to Christmas cracker puns, she explains, the positive effect is more probable to be caused not by the joke 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 chuckle together."

The Search for the Perfect Festive Pun

Will we ever find the ultimate joke?

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

In 2001, a professor set up a research project for the planet's most humorous gag.

More than tens of thousands of gags later, with ratings lodged by 350,000 participants around the world, he has a clearer understanding than many as to what works and what fails.

The perfect festive cracker joke must be short, he says.

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

The increasingly "terrible" the gag, he states the better.

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

"The fascinating part about the Christmas cracker jokes is that not one person considers them humorous.

"That's a common experience at the table and I think it's wonderful."