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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 storage facility in London.

We're at a humor-evaluation session with a company that produces supplies for gatherings. Its catalogue features festive crackers.

The firm's owner smiles, almost apologetically at the joke. But the joke has made the cut and will appear in upcoming crackers.

"You measure the gag by the number of groans and the intensity of the groans at the table," she explains.

The key to a good Christmas cracker pun is not the identical as a stand-up gag per se. It is all about the setting - in this case, the communal amusement of the holiday dinner table with elders, children and possibly neighbours.

"You want the joke to be something that brings the eight-year-old in harmony with the grandparent," she adds.

The Neuroscience Behind Shared Amusement

Gathering to enjoy shared amusement is not only nothing new, experts argue, it is likely to be pre-human.

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

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

Scientists have found that a absence of such interactions can significantly harm mental and physical well-being.

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

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

"You're not just chuckling at a silly joke with a Christmas cracker," she says. "You are actually doing a lot of the truly important task of building, preserving the connections you have with the people you love."

Which Happens Inside the Mind?

But what is truly happening within the mind when we hear a joke?

A tremendous amount happens in response to comedy, it transpires.

Employing brain scanning technology, a type of brain scanner which indicates which areas of the mind are more active, researchers have been able to chart the regions that receive more blood flow.

The research involves scanning the brains of healthy subjects and then subjecting them to a database of humorous words, accompanied by either a non-emotional sound, or recorded chuckles.

"In the scanner we got a really fascinating activation pattern of activation," says the neuroscientist.

A joke stimulates not just the areas of the brain in charge of hearing and understanding language, but also brain regions involved in both preparation and initiating movement and those linked to vision and memory.

Put these elements together, and people listening to a pun have a sophisticated series of brain reactions that underpin the amusement we hear.

The Contagious Power of Chuckles

Researchers discovered that when a humorous phrase is paired with laughter there is a greater response in the brain than the same word 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 grin or a chuckle," she says.

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

Amusement, says the professor, can be contagious.

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

"You laugh harder when you are familiar with others," she says, "and laughter increases more when you like them or love them."

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

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

The Quest for the Perfect Festive Pun

Is it possible to find the perfect joke?

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

Years ago, a professor established a scientific project for the planet's funniest gag.

More than tens of thousands of gags submitted, with ratings provided by hundreds of thousands of participants globally, he has a clearer understanding than most as to what works and what does not.

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

"But they also be bad jokes, puns that cause us to groan," he continues.

The increasingly "terrible" the joke, he says the better.

"This is because if nobody finds it funny – it's the joke's shortcoming, not your own.

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

"That's a common experience around the gathering and I think it's wonderful."