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"How much did Santa's sled cost? Nothing, it was on the house."

This quip is met by groans that resonate through a warehouse in London.

We're at a joke-testing meeting with a company that makes products for social events. Its catalogue includes festive crackers.

The firm's owner grins, almost apologetically at the joke. But the joke has been selected and will appear in upcoming crackers.

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

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

"The goal is for the gag to be a thing that unites the eight-year-old together with the grandparent," she states.

The Science Behind Communal Laughter

Coming together to enjoy shared amusement is not only ancient, scientists say, it is probably to be pre-human.

"So when you are chuckling with people at the Christmas table you are dropping into what's very likely a truly primordial mammalian play vocalisation," says a neuroscience expert.

Communal laughter, she says, helps forge and strengthen social bonds between people.

Scientists have found that a lack of such interactions can significantly harm mental and physical health.

"Those you converse with, and laugh with, it leads to enhanced amounts of 'happy chemical' uptake," she adds.

Endorphins are the body's "feel-good compounds" and are produced both to reduce tension and discomfort and in reaction to pleasurable experiences, such as chuckling with loved ones over a truly awful festive cracker gag.

"It's not simply chuckling at a foolish pun with a holiday cracker," the expert says. "You are actually performing a lot of the really important work of building, preserving the connections you have with those you love."

What Happens Inside the Mind?

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

A tremendous amount happens in reaction to humour, it transpires.

Using functional magnetic resonance imaging (fMRI), a type of neural imager which indicates which parts of the brain are working harder, scientists have been able to map the areas that get more blood flow.

Testing involves scanning the minds of volunteer participants and then subjecting them to a database of funny words, accompanied by either a neutral sound, or recorded laughter.

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

A joke stimulates not just the parts of the brain in charge of auditory processing and understanding language, but also neural regions associated with both preparation and starting movement and those linked to vision and recall.

Combine these elements together, and people hearing a joke have a sophisticated series of neural reactions that support the laughter we experience.

The Infectious Power of Laughter

Researchers found that when a humorous phrase is paired with chuckles there is a stronger response in the brain than the identical word when followed by a neutral sound.

"This was in areas of the mind that you would use to move your face into a grin or a chuckle," the professor says.

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

Amusement, according to the professor, can be infectious.

So what does this imply for the laughter found at a holiday table?

"People laugh more when you are familiar with others," she says, "and you laugh further when you like them or care for them."

When it comes to Christmas cracker jokes, she says, the positive factor is more likely to be triggered not by the joke itself, but from the reaction to it.

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

The Quest for the Perfect Cracker Joke

Is it possible to discover the ultimate joke?

Likely not, but that has not prevented researchers from attempting to.

In 2001, a professor established a scientific project for the planet's funniest gag.

More than tens of thousands of gags later, with scores provided by 350,000 people around the world, he has a clearer understanding than most as to what succeeds and what fails.

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

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

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

"This is because if no-one laughs – it's the gag's fault, not your own.

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

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