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"How much did Santa's sleigh cost? Zero, it was on the house."

This quip is met by moans that resonate through a storage facility in London.

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

The company's founder smiles, nearly sheepishly at the gag. But the pun has been selected and will feature in future crackers.

"You measure the joke by the volume of groans and the loudness of the groans around the table," she says.

The key to a good holiday cracker joke is not the same as a stand-up gag per se. It is entirely about the context - in this case, the shared laughter of the Christmas meal with elders, kids and possibly friends.

"The goal is for the joke to be something that unites the eight-year-old together with the 80-year-old," she adds.

The Neuroscience Behind Shared Amusement

Coming together to enjoy shared laughter is not only nothing new, experts say, it is probably to be pre-human.

"Therefore when you are chuckling with others at the Christmas dinner you are dropping into what's very likely a really ancient mammal social sound," explains a neuroscience expert.

Shared laughter, she explains, helps forge and strengthen social connections between individuals.

Scientists have found that a lack of these interactions can seriously harm both psychological and bodily well-being.

"Those you talk to, and share laughter with, it results in increased levels of endorphin release," the professor 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 friends over a particularly awful festive cracker gag.

"You're not just chuckling at a foolish joke with a holiday cracker," the expert states. "You are actually performing a lot of the really vital task of making, maintaining the connections you have with those you love."

What Happens Inside the Mind?

But what is truly happening inside the brain when we hear a gag?

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

Employing brain scanning technology, a kind of neural imager which shows which parts of the mind are working harder, researchers have been able to map the regions that get more blood.

The research entails imaging the brains of healthy subjects and then subjecting them to a collection of funny words, accompanied by either a neutral sound, or pre-recorded chuckles.

"In the scanner we observed a very interesting activation pattern of neural activity," notes the professor.

A gag activates not just the areas of the mind responsible for hearing and interpreting speech, but also brain areas involved in both preparation and starting motion and those linked to sight and recall.

Put these elements together, and people hearing a joke have a complex set of neural reactions that support the amusement we hear.

The Contagious Power of Chuckles

Scientists discovered that when a humorous word is paired with chuckles there is a stronger response in the mind than the identical phrase when accompanied by a non-emotional sound.

"This activation occurred in parts of the brain that you would use to contort your expression into a grin or a laugh," she explains.

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

Laughter, according to the expert, can be infectious.

So what does this mean for the laughter found at a Christmas table?

"People laugh more when you know people," she notes, "and laughter increases more when you are fond of them or love them."

When it comes to Christmas cracker puns, she explains, the feel-good factor is more likely to be caused not by the joke in itself, but from the reaction to it.

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

The Quest for the Ideal Festive Pun

Will we ever find the perfect gag?

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

In 2001, a psychologist set up a research project for the planet's funniest joke.

More than tens of thousands of gags later, with ratings lodged by hundreds of thousands of participants globally, he has a better understanding than most as to what works and what fails.

The perfect festive cracker pun needs to be brief, he says.

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

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

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

"What's interesting about the holiday cracker jokes is that none of us considers them humorous.

"It creates a shared moment around the gathering and I believe it's wonderful."