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

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

This describes a humor-evaluation session with a company that makes supplies for gatherings. Its repertoire features festive crackers.

The company's owner grins, almost apologetically at the gag. But the pun has been selected and will feature in upcoming crackers.

"The success is gauged by the gag by the volume of groans and the intensity of the groans at the table," she explains.

The key to a good Christmas cracker pun is not the same as a stand-up joke per se. It is entirely about the setting - in this instance, the communal amusement of the holiday dinner table with elders, kids and potentially friends.

"The goal is for the gag to be something that brings the child together with the 80-year-old," she states.

The Neuroscience Behind Shared Laughter

Coming together to enjoy shared laughter is not only ancient, experts say, it is probably to be older than humanity.

"Therefore when you are chuckling with others at the Christmas table you are dropping into what's very likely a truly ancient mammal social vocalisation," explains a neuroscience expert.

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

Scientists have discovered that a lack of these interactions can seriously damage mental and physical health.

"The people you converse with, and laugh with, it results in enhanced amounts of 'happy chemical' uptake," the professor adds.

These natural chemicals are the brain's "happy chemicals" and are released both to alleviate tension and discomfort and in response to enjoyable experiences, such as laughing with loved ones over a particularly terrible Christmas cracker joke.

"You're not just chuckling at a foolish pun with a holiday cracker," the expert says. "You are in fact performing a lot of the really vital work of building, preserving the connections you have with the people you care about."

Which Happens Inside the Brain?

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

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

Using brain scanning technology, a type of brain scanner which indicates which parts of the brain are more active, scientists have been able to map the areas that get more blood.

Testing entails imaging the minds of volunteer subjects and then exposing them to a database of funny phrases, paired with either a neutral sound, or recorded laughter.

"During the study we observed a really fascinating activation pattern of neural activity," notes the professor.

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

Put all of this as a whole, and individuals listening to a pun have a sophisticated series of brain reactions that support the laughter we experience.

The Infectious Nature of Laughter

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

"This activation occurred in parts of the mind that you would use to move your face into a grin or a laugh," the professor says.

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

Amusement, says the professor, can be contagious.

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

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

When it comes to Christmas cracker puns, she says, the feel-good effect is more probable to be caused not by the joke itself, but from the response to it.

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

The Quest for the Perfect Festive Pun

Is it possible to discover the perfect gag?

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

In 2001, a psychologist established a research search for the planet's most humorous gag.

More than tens of thousands of jokes later, with ratings provided by hundreds of thousands of participants globally, he has a clearer understanding than many as to what succeeds and what does not.

The perfect Christmas cracker joke needs to be short, he explains.

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

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

"The reason is that if nobody finds it funny – it's the gag's shortcoming, not yours.

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

"It creates a common moment at the gathering and I believe it's wonderful."