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

This one-liner is greeted with groans that echo through a storage facility in London.

This describes a humor-evaluation meeting with a firm that makes products for social events. Its repertoire features Christmas crackers.

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

"The success is gauged by the joke by the volume of moans and the intensity of the groans at the table," the founder says.

The secret to a good holiday cracker pun is not the identical as a good joke in itself. It is entirely about the context - in this case, the shared amusement of the holiday meal with grandparents, kids and possibly neighbours.

"You want the gag to be something that unites the child in harmony with the grandparent," she adds.

The Neuroscience Behind Shared Amusement

Coming together to experience communal laughter is not only ancient, experts argue, it is probably to be older than humanity.

"Therefore when you are chuckling with people at the holiday dinner you are dropping into what's almost certainly a truly primordial mammal social sound," says a neuroscience expert.

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

Researchers have discovered that a lack of these social exchanges can significantly harm mental and physical health.

"Those you converse with, and laugh with, it results in increased levels of 'happy chemical' release," she adds.

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

"It's not simply laughing at a silly pun with a Christmas cracker," the expert says. "You are actually doing a lot of the truly important work of making, maintaining the connections you have with those you love."

Which Happens Inside the Mind?

But what is truly taking place within the brain when we listen to a gag?

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

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

The research involves scanning the brains of healthy participants and then subjecting them to a database of funny words, paired with either a non-emotional sound, or recorded chuckles.

"In the scanner we got a very interesting pattern of neural activity," notes the neuroscientist.

A joke activates not just the areas of the mind in charge of auditory processing and interpreting language, but also neural areas involved in both preparation and initiating motion and those linked to vision and recall.

Put these elements together, and people listening to a joke have a sophisticated set of brain responses that underpin the amusement we experience.

The Infectious Nature of Laughter

Scientists discovered that when a humorous phrase is combined with laughter there is a greater response in the mind than the identical word when accompanied by a non-emotional sound.

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

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

Amusement, according to the professor, can be infectious.

So what does this mean for the laughter heard around a holiday table?

"You laugh more when you know others," she says, "and you laugh further when you like them or care for them."

When it comes to Christmas cracker puns, she explains, the feel-good factor is more probable to be triggered not by the gag itself, but from the reaction to it.

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

The Quest for the Perfect Festive Pun

Is it possible to find the ultimate joke?

Probably not, but that has not prevented experts from trying to.

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

More than 40,000 jokes later, with scores provided by 350,000 participants around the world, he has a clearer understanding than many as to what works and what does not.

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

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

The more "terrible" the joke, he says the better.

"This is because if no-one finds it funny – it's the joke's shortcoming, not your own.

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

"It creates a shared moment around the gathering and I think it's wonderful."