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

This one-liner is met by groans that resonate through a warehouse in London.

We're at a humor-evaluation meeting with a firm that makes products for social events. Its repertoire features Christmas crackers.

The firm's owner smiles, almost sheepishly at the gag. But the joke has been selected and will appear in future crackers.

"You measure the joke by the number of groans and the loudness of the groans around the table," the founder explains.

The key to a good Christmas cracker pun is not the identical as a stand-up joke per se. It is all about the context - in this case, the shared amusement of the holiday dinner table with grandparents, kids and potentially friends.

"You want the gag to be a thing that brings the child together with the 80-year-old," she adds.

The Neuroscience Of Communal Laughter

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

"Therefore when you are chuckling with others around the Christmas table you are dropping into what's almost certainly a really ancient mammalian social vocalisation," says a neuroscience expert.

Communal laughter, she explains, aids in make and maintain social bonds between people.

Scientists have discovered that a absence of these interactions can seriously damage mental and physical health.

"Those you talk to, and share laughter with, it results in increased levels of endorphin release," the professor continues.

Endorphins are the brain's "happy chemicals" and are produced both to alleviate stress and pain and in reaction to pleasurable activities, such as laughing with friends over a particularly terrible Christmas cracker gag.

"You're not just laughing at a foolish 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 those you love."

Which Happens In the Mind?

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

A tremendous amount occurs in response to humour, it transpires.

Employing functional magnetic resonance imaging (fMRI), a kind of brain scanner which indicates which areas of the mind are working harder, scientists have been able to map the regions that get more blood flow.

The research entails imaging the minds of healthy participants and then exposing them to a collection of funny words, accompanied by either a neutral sound, or recorded laughter.

"During the study we got a very fascinating pattern of neural activity," says the professor.

A joke stimulates not just the areas of the mind responsible for hearing and understanding language, but also neural areas involved in both planning and starting movement and those linked to vision and memory.

Combine all of this together, and people hearing a pun have a complex set of brain reactions that underpin the amusement we hear.

The Infectious Nature of Laughter

Scientists found that when a humorous word is paired with laughter there is a stronger reaction in the mind than the identical word when followed by a non-emotional sound.

"This activation occurred in areas of the brain that you would employ to move your expression into a smile or a laugh," she says.

It means we are not just responding to funny words, they are responding to the laughter that accompanies them.

Laughter, according to the professor, can be contagious.

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

"You laugh more when you know others," she notes, "and you laugh further when you like 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 itself, but from the response to it.

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

The Quest for the Perfect Cracker Joke

Will we ever find the perfect gag?

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 joke.

More than tens of thousands of jokes submitted, with scores provided by hundreds of thousands of participants around the world, he has a clearer understanding than many as to what works and what does not.

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

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

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

"The reason is that if nobody laughs – it's the joke's shortcoming, not your own.

"What's interesting about the Christmas cracker puns is that not one person considers them funny.

"That's a common experience at the gathering and I believe it's wonderful."