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

This one-liner is met by groans that resonate through a storage facility in London.

We're at a humor-evaluation meeting with a company that makes products for social events. Its catalogue features festive crackers.

The company's founder smiles, almost apologetically at the joke. But the pun has made the cut and will appear in upcoming crackers.

"You measure the gag by the volume of moans and the intensity of the groans at the table," the founder explains.

The key to a great Christmas cracker joke is not the identical as a stand-up gag in itself. It is entirely about the setting - in this instance, the communal laughter of the holiday dinner table with grandparents, children and possibly friends.

"You want the gag to be something that brings the eight-year-old together with the grandparent," she states.

The Neuroscience Behind Shared Laughter

Coming together to experience shared amusement is not only ancient, experts argue, it is likely to be older than humanity.

"Therefore when you are laughing with people at the holiday table you are engaging in what's very likely a really ancient mammal social vocalisation," says a neuroscience expert.

Communal laughter, she explains, aids in make and maintain social bonds between individuals.

Scientists have discovered that a lack of these interactions can significantly harm mental and physical well-being.

"Those you talk to, and share laughter with, it leads to enhanced levels of endorphin release," she adds.

Endorphins are the body's "happy chemicals" and are produced both to reduce stress and pain and in reaction to enjoyable experiences, such as laughing with loved ones over a truly terrible festive cracker joke.

"It's not simply chuckling at a silly pun with a holiday cracker," the expert states. "You are in fact performing a lot of the truly vital task of building, preserving the connections you have with those you care about."

Which Happens In the Mind?

But what is truly taking place inside the mind when we hear a joke?

An awful lot occurs in reaction to comedy, it transpires.

Using brain scanning technology, a type of brain scanner which shows which parts of the mind are more active, researchers have been able to chart the areas that receive more blood flow.

Testing involves imaging the brains of volunteer participants and then subjecting them to a database of funny words, accompanied by either a non-emotional sound, or pre-recorded chuckles.

"In the scanner we observed a very fascinating pattern of neural activity," says the professor.

A gag stimulates not just the areas of the brain responsible for hearing and understanding language, but also neural areas involved in both planning and starting movement and those involved in sight and recall.

Put these elements together, and people listening to a joke have a complex set of neural reactions that support the laughter we experience.

The Infectious Power of Laughter

Scientists found that when a humorous word is paired with laughter there is a stronger reaction in the brain than the same phrase when accompanied by a neutral sound.

"This was in parts of the brain that you would use to contort your expression into a smile or a laugh," she says.

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

Laughter, according to the expert, can be contagious.

So what does this mean for the laughter found at a Christmas table?

"You laugh more when you know others," she notes, "and laughter increases more when you like them or love them."

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

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

The Search for the Ideal Festive Pun

Is it possible to discover the perfect joke?

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

In 2001, a psychologist established a scientific search for the world's most humorous joke.

More than tens of thousands of jokes later, with ratings provided by hundreds of thousands of people globally, he has a clearer understanding than most as to what works and what fails.

The ideal Christmas cracker joke must be short, he says.

"They must also be bad jokes, puns that cause us to moan," he adds.

The increasingly "terrible" the gag, he states the better.

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

"What's interesting about the holiday cracker jokes is that none of us find them humorous.

"It creates a shared moment around the gathering and I believe it's lovely."