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"What was the price did Father Christmas's sleigh cost? Zero, it was on the house."

This one-liner is met by groans that resonate through a storage facility in the capital.

This describes a joke-testing session with a company that produces products for gatherings. Its catalogue includes Christmas crackers.

The firm's founder grins, nearly apologetically at the gag. But the pun has made the cut and will feature in upcoming crackers.

"The success is gauged by the joke by the volume of moans and the loudness 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 context - in this instance, the shared laughter of the holiday meal with elders, children and potentially neighbours.

"You want the gag to be something that brings the child in harmony with the grandparent," she adds.

The Science Behind Communal Laughter

Gathering to experience communal laughter is not only ancient, experts say, it is probably to be older than humanity.

"Therefore when you are laughing with people around the Christmas dinner you are dropping into what's very likely a really primordial mammal social sound," says a professor.

Communal amusement, she explains, helps forge and strengthen social bonds between people.

Researchers have found that a lack of these interactions can seriously damage mental and physical well-being.

"The people you talk to, and share laughter with, it leads to enhanced levels of 'happy chemical' release," she adds.

Endorphins are the brain's "feel-good compounds" and are released both to alleviate tension and discomfort and in reaction to pleasurable activities, such as laughing with friends over a particularly terrible Christmas cracker joke.

"It's not simply chuckling at a foolish joke with a Christmas cracker," she states. "You are actually doing a lot of the really vital task of building, preserving the connections you have with those you love."

What Happens Inside the Mind?

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

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

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

The research involves imaging the minds of healthy subjects and then exposing them to a collection of funny words, paired with either a neutral sound, or pre-recorded laughter.

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

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

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

The Contagious Nature of Chuckles

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

"This activation occurred in parts of the mind that you would employ to move your face into a grin or a chuckle," the professor says.

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

Amusement, says the expert, can be contagious.

So what does this imply for the chuckles heard at a holiday table?

"You laugh harder when you know people," she says, "and laughter increases further when you like them or love them."

When it comes to Christmas cracker jokes, she says, the feel-good effect is more probable to be caused not by the joke itself, but from the response to it.

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

The Quest for the Perfect Cracker Joke

Will we ever find the perfect joke?

Probably not, but that has not prevented researchers from trying to.

Years ago, a professor established a scientific search for the planet's funniest joke.

More than tens of thousands of jokes submitted, with ratings provided by hundreds of thousands of people around the world, he has a better idea than most as to what works and what does not.

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

"They must also be poor jokes, puns that cause us to moan," he adds.

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

"This is because if nobody finds it funny – it's the gag's fault, not yours.

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

"That's a shared experience around the table and I believe it's lovely."