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

This joke is met by moans that echo through a warehouse in the capital.

This describes a joke-testing meeting with a company that makes products for social events. Its catalogue features festive crackers.

The firm's owner grins, nearly apologetically at the joke. But the joke has been selected and will feature in future crackers.

"The success is gauged by the joke by the number of moans and the intensity of the groans around the table," the founder explains.

The secret to a good Christmas cracker pun is not the identical as a stand-up gag in itself. It is entirely about the setting - in this case, the communal amusement of the holiday dinner table with grandparents, children and potentially friends.

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

The Neuroscience Of Communal Laughter

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

"So when you are laughing with others at the holiday dinner you are dropping into what's almost certainly a really ancient mammal play vocalisation," says a professor.

Shared laughter, she explains, aids in make and maintain social bonds between individuals.

Scientists have found that a lack of such social exchanges can seriously harm both psychological and bodily well-being.

"The people you talk to, and share laughter with, it leads to enhanced levels of endorphin uptake," the professor adds.

Endorphins are the brain's "happy chemicals" and are released both to reduce tension and discomfort and in reaction to enjoyable experiences, such as laughing with friends over a truly terrible festive cracker gag.

"It's not simply chuckling at a silly pun with a holiday cracker," the expert says. "You are actually performing a lot of the truly important task of building, preserving the social bonds you have with the people you care about."

What Happens In the Brain?

But what is actually taking place within the mind when we hear a joke?

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

Using functional magnetic resonance imaging (fMRI), a type of neural imager which shows which areas of the brain are working harder, researchers have been able to chart the regions that get more blood.

The research involves imaging the brains of volunteer participants and then exposing them to a database of humorous words, accompanied by either a neutral sound, or pre-recorded chuckles.

"During the study we got a really interesting activation pattern of activation," says the neuroscientist.

A joke activates not just the areas of the mind responsible for auditory processing and interpreting speech, but also brain areas involved in both planning and starting motion and those involved in sight and recall.

Combine all of this as a whole, and people listening to a joke have a sophisticated series of neural reactions that support the laughter we hear.

The Contagious Power of Chuckles

Researchers discovered that when a funny phrase is paired with chuckles there is a stronger reaction in the brain than the same word when followed by a non-emotional sound.

"This was in areas of the brain that you would employ to move your face into a grin or a laugh," she explains.

It indicates people are not just reacting to funny words, they are responding to the amusement that accompanies them.

Amusement, according to the expert, can be contagious.

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

"You laugh harder when you know people," she notes, "and you laugh further when you are fond of 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 in itself, but from the reaction to it.

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

The Search for the Ideal Cracker Joke

Will we ever discover the ultimate gag?

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

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

More than 40,000 jokes later, with scores lodged by 350,000 people around the world, he has a better idea than many as to what works and what fails.

The ideal Christmas cracker pun needs to be brief, he explains.

"But they also need to be poor gags, puns that cause us to groan," he adds.

The more "awful" the joke, he states the better.

"The reason is that if nobody finds it funny – it's the gag's fault, not yours.

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

"That's a shared moment at the gathering and I think it's wonderful."