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"What was the price did Santa's sled cost? Zero, it was on the house."

This quip is greeted with groans that resonate through a storage facility in London.

We're at a humor-evaluation session with a company that makes supplies for gatherings. Its repertoire features Christmas crackers.

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

"The success is gauged by the gag by the number of moans and the intensity of the groans around the table," she says.

The key to a good Christmas cracker joke is not the identical as a stand-up joke in itself. It is all about the setting - in this instance, the shared laughter of the Christmas meal with elders, children and potentially friends.

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

The Neuroscience Behind Communal Amusement

Coming together to enjoy communal laughter is not only nothing new, scientists argue, it is probably to be older than humanity.

"So when you are laughing with others at the holiday table you are engaging in what's very likely a truly ancient mammal social sound," explains a professor.

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

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

"The people you talk to, and laugh with, it leads to enhanced levels of endorphin release," the professor continues.

Endorphins are the body's "happy chemicals" and are produced both to alleviate tension and discomfort and in reaction to pleasurable experiences, such as laughing with loved ones over a particularly terrible festive cracker gag.

"It's not simply chuckling at a silly pun with a holiday cracker," she says. "You are actually performing a lot of the truly vital work of building, preserving the connections you have with the people you care about."

Which Happens Inside the Mind?

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

A tremendous amount occurs in reaction to humour, it turns out.

Using functional magnetic resonance imaging (fMRI), a type of brain scanner which indicates which parts of the brain are more active, researchers have been able to chart the regions that receive more blood flow.

The research involves scanning the brains of healthy participants and then exposing them to a collection of funny phrases, paired with either a non-emotional sound, or pre-recorded laughter.

"During the study we got a very interesting pattern of neural activity," notes the neuroscientist.

A joke activates not just the parts of the brain responsible for auditory processing and understanding speech, but also brain areas associated with both planning and initiating movement and those involved in sight and recall.

Put all of this as a whole, and people listening to a pun have a sophisticated set of brain reactions that underpin the amusement we experience.

The Infectious Nature of Chuckles

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

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

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

Amusement, says the professor, can be infectious.

So what does this mean for the laughter found around a holiday gathering?

"You laugh more when you are familiar with people," she says, "and laughter increases more when you are fond of them or care for them."

When it comes to Christmas cracker jokes, she explains, the positive effect is more likely to be caused not by the gag itself, but from the reaction to it.

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

The Quest for the Perfect Cracker Joke

Will we ever discover the perfect joke?

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

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

Over tens of thousands of jokes submitted, with scores lodged by 350,000 participants globally, he has a clearer idea than many as to what succeeds and what does not.

The perfect Christmas cracker pun must be short, he says.

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

The more "awful" the joke, he says the more effective.

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

"The fascinating part about the Christmas cracker puns is that none of us considers them funny.

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