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

This quip is met by moans that echo through a storage facility in London.

We're at a joke-testing meeting with a firm that produces products for gatherings. Its catalogue includes festive crackers.

The firm's founder grins, nearly sheepishly at the gag. But the pun has been selected and will feature in upcoming crackers.

"The success is gauged by the joke by the volume of moans and the loudness of the groans around the table," she explains.

The key to a good Christmas cracker joke is not the same as a stand-up joke per se. It is entirely about the setting - in this instance, the shared amusement of the Christmas meal with grandparents, children and potentially friends.

"You want the joke to be a thing that unites the eight-year-old together with the grandparent," she adds.

The Neuroscience Of Communal Amusement

Gathering to experience shared laughter is not only nothing new, scientists say, it is probably to be older than humanity.

"Therefore when you are laughing with people at the Christmas dinner you are engaging in what's very likely a truly ancient mammalian social sound," explains a professor.

Communal laughter, she says, helps forge and strengthen social connections between individuals.

Researchers have found that a lack of such social exchanges can significantly harm mental and physical well-being.

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

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

"It's not simply laughing at a silly joke with a Christmas cracker," she says. "You are in fact performing a lot of the truly important task of building, preserving the social bonds you have with those you care about."

Which Happens In the Brain?

But what is actually happening inside the brain when we listen to a gag?

An awful lot occurs in reaction to comedy, it turns out.

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

The research entails imaging the minds of healthy participants and then exposing them to a database of humorous phrases, paired with either a neutral sound, or recorded laughter.

"In the scanner we observed a very fascinating pattern of neural activity," notes the neuroscientist.

A gag stimulates not just the parts of the brain responsible for auditory processing and interpreting language, but also neural regions associated with both planning and starting motion and those involved in vision and recall.

Put all of this as a whole, and people hearing a joke have a complex series of brain reactions that support the amusement we experience.

The Contagious Power of Laughter

Researchers discovered that when a funny word is combined with chuckles there is a stronger reaction in the brain than the same word when accompanied by a neutral sound.

"This was in areas of the mind that you would employ to move your expression into a grin or a laugh," the professor says.

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

Laughter, according to the professor, can be infectious.

So what does this imply for the laughter heard at a Christmas gathering?

"You laugh harder when you know others," she says, "and laughter increases further when you are fond of them or care for them."

When it comes to festive cracker jokes, she says, the feel-good factor is more probable to be triggered not by the joke in itself, but from the response to it.

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

The Search for the Perfect Cracker Joke

Is it possible to find the perfect joke?

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

In 2001, a professor set up a research search for the world's most humorous gag.

Over tens of thousands of jokes later, with ratings provided by 350,000 participants around the world, he has a clearer understanding than most as to what succeeds and what does not.

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

"But they also need to be poor gags, jokes that make us moan," he continues.

The increasingly "awful" the gag, he states the more effective.

"The reason is that if no-one finds it funny – it's the gag's shortcoming, not yours.

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

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