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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 resonate through a storage facility in the capital.

We're at a joke-testing meeting with a company that produces products for social events. Its repertoire features festive crackers.

The firm's founder smiles, nearly sheepishly at the gag. But the pun has made the cut and will feature in future crackers.

"You measure the gag by the number of moans and the intensity of the groans around 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 entirely about the context - in this case, the communal amusement of the holiday meal with grandparents, kids and possibly friends.

"You want the joke to be a thing that unites the child together with the grandparent," she states.

The Neuroscience Of Communal Laughter

Coming together to enjoy communal laughter is not only ancient, experts argue, it is likely to be pre-human.

"Therefore when you are chuckling with people at the holiday dinner you are dropping into what's very likely a truly ancient mammalian social vocalisation," says a professor.

Shared amusement, she says, aids in forge and strengthen social bonds between people.

Scientists have found that a absence of such interactions can significantly damage mental and physical well-being.

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

These natural chemicals are the body's "feel-good compounds" and are released both to alleviate tension and discomfort and in response to pleasurable activities, such as chuckling with loved ones over a truly terrible festive cracker joke.

"You're not just chuckling at a foolish pun with a Christmas cracker," the expert says. "You are in fact doing a lot of the truly vital work of building, preserving the social bonds you have with those you care about."

Which Happens In the Brain?

But what is actually happening within the mind when we listen to a gag?

An awful lot occurs in response to humour, it turns out.

Employing functional magnetic resonance imaging (fMRI), a kind of brain scanner which indicates which areas of the brain are more active, researchers have been able to map the regions that receive more blood flow.

Testing entails imaging the minds of healthy subjects and then exposing them to a database of funny words, accompanied by either a neutral sound, or pre-recorded laughter.

"In the scanner we got a really interesting pattern of neural activity," says the professor.

A joke activates not just the areas of the brain in charge of hearing and interpreting language, but also neural regions involved in both preparation and initiating motion and those involved in sight and memory.

Put all of this as a whole, and individuals hearing a joke have a sophisticated series of neural responses that support the amusement we hear.

The Infectious Nature of Laughter

Researchers found that when a humorous phrase is combined with chuckles there is a stronger reaction in the brain than the identical word when followed by a non-emotional sound.

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

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

Amusement, according to the expert, can be contagious.

So what does this mean for the chuckles found around a Christmas gathering?

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

When it comes to festive cracker puns, she explains, the positive effect is more probable to be caused not by the joke in itself, but from the reaction to it.

"The laughter is key. The joke is the dreadful Christmas cracker pun, and it's just a reason to chuckle together."

The Search for the Ideal Cracker Joke

Is it possible to discover the ultimate gag?

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

Years ago, a professor established a scientific project for the world's most humorous joke.

Over tens of thousands of gags later, with scores lodged by hundreds of thousands of participants globally, he has a clearer idea than many as to what works and what does not.

The ideal festive cracker joke must be short, he explains.

"They must also need to be bad gags, jokes that cause us to moan," he continues.

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

"This is because if nobody laughs – it's the gag's fault, not your own.

"The fascinating part about the Christmas cracker puns is that not one person considers them funny.

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