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"How much did Santa's sled cost? Nothing, it was on the house."

This joke is greeted with groans that resonate through a warehouse in London.

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

The company's owner grins, nearly apologetically at the joke. But the joke has made the cut and will feature in future crackers.

"You measure the gag by the number of moans and the loudness of the groans at the table," the founder says.

The key to a great Christmas cracker pun is not the identical as a good joke per se. It is entirely about the context - in this instance, the shared amusement of the holiday dinner table with elders, kids and potentially neighbours.

"You want the gag to be a thing that unites the child in harmony with the 80-year-old," she states.

The Neuroscience Behind Shared Laughter

Coming together to enjoy shared amusement is not only nothing new, experts argue, it is probably to be pre-human.

"So when you are laughing with people around the holiday dinner you are dropping into what's almost certainly a truly primordial mammalian play vocalisation," explains a professor.

Communal amusement, she explains, aids in make and maintain social connections between people.

Researchers have discovered that a lack of these social exchanges can seriously damage both psychological and bodily well-being.

"The people you converse with, and share laughter with, it leads to enhanced amounts of 'happy chemical' release," she continues.

Endorphins are the body's "happy chemicals" and are produced both to reduce tension and discomfort and in reaction to pleasurable activities, such as laughing with friends over a truly awful festive cracker joke.

"You're not just chuckling at a silly pun with a Christmas cracker," she says. "You are in fact doing a lot of the really important task of making, maintaining the connections you have with the people you care about."

What Occurs In the Brain?

But what is truly happening inside the mind when we listen to a joke?

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

Employing functional magnetic resonance imaging (fMRI), a type of neural imager which shows which parts of the mind are more active, researchers have been able to chart the areas that get more blood flow.

Testing entails scanning the minds of volunteer subjects and then subjecting them to a collection of funny words, paired with either a neutral sound, or pre-recorded laughter.

"During the study we observed a very interesting pattern of activation," notes the neuroscientist.

A gag stimulates not just the parts of the brain in charge of hearing and understanding speech, but also brain regions involved in both planning and starting motion and those linked to sight and memory.

Put these elements as a whole, and people listening to a joke have a sophisticated series of brain responses that support the amusement we experience.

The Infectious Nature of Laughter

Scientists found that when a funny phrase is combined with laughter there is a greater response in the mind than the identical phrase when followed by a non-emotional sound.

"This activation occurred in areas of the brain that you would employ to contort your expression into a grin or a chuckle," the professor says.

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

Laughter, says the expert, can be infectious.

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

"You laugh harder when you know people," she says, "and you laugh more when you are fond of them or care for them."

When it comes to festive cracker jokes, she explains, the feel-good effect is more likely to be triggered not by the gag in itself, but from the reaction to it.

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

The Quest for the Perfect Cracker Joke

Is it possible to discover the ultimate joke?

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

In 2001, a psychologist established a research project for the world's most humorous joke.

More than tens of thousands of jokes later, with scores provided by 350,000 participants globally, he has a better understanding than most as to what succeeds and what fails.

The perfect festive cracker pun must be brief, he says.

"They must also need to be poor gags, jokes that cause us to groan," he adds.

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

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

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

"It creates a shared moment at the gathering and I think it's lovely."