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"How much did Santa's sled cost? Nothing, 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 company that makes supplies for social events. Its catalogue features festive crackers.

The firm's owner grins, almost apologetically at the gag. But the pun has made the cut and will feature in upcoming crackers.

"You measure the joke by the volume of groans and the intensity of the groans at the table," she says.

The secret to a great holiday cracker joke is not the same as a stand-up gag in itself. It is entirely about the setting - in this case, the communal amusement of the holiday dinner table with elders, kids and possibly friends.

"You want the joke to be a thing that brings the eight-year-old together with the 80-year-old," she states.

The Science Behind Shared Amusement

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

"Therefore when you are laughing with people around the holiday dinner you are engaging in what's almost certainly a really ancient mammal play vocalisation," explains a neuroscience expert.

Shared laughter, she explains, helps make and maintain social connections between people.

Researchers have discovered that a lack of such interactions can seriously harm mental and physical well-being.

"Those you converse with, and laugh with, it leads to enhanced levels of 'happy chemical' release," the professor continues.

Endorphins are the brain's "happy chemicals" and are produced both to alleviate stress and pain and in response to pleasurable experiences, such as laughing with loved ones over a particularly terrible festive cracker gag.

"You're not just laughing at a foolish joke with a holiday cracker," the expert says. "You are in fact performing a lot of the really vital work of building, preserving the social bonds you have with those you love."

What Occurs Inside the Mind?

But what is truly taking place inside the brain when we hear a gag?

A tremendous amount occurs in response to humour, it transpires.

Employing functional magnetic resonance imaging (fMRI), a kind of neural imager which shows which areas of the brain are more active, scientists have been able to map the areas that receive more blood flow.

The research involves imaging the minds of volunteer subjects and then subjecting them to a database of funny phrases, paired with either a neutral sound, or recorded chuckles.

"In the scanner we observed a really interesting pattern of activation," notes the professor.

A joke activates not just the parts of the mind responsible for hearing and interpreting speech, but also neural areas associated with both planning and initiating movement and those linked to vision and recall.

Put these elements as a whole, and individuals hearing a pun have a sophisticated series of neural responses that support the amusement we experience.

The Contagious Nature of Chuckles

Scientists discovered that when a funny phrase is paired with chuckles there is a stronger response in the mind than the same word when followed by a neutral sound.

"This activation occurred in parts of the mind that you would employ to move your expression into a grin or a laugh," she explains.

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

Laughter, says the expert, can be contagious.

So what does this mean for the chuckles heard around a Christmas gathering?

"You laugh harder when you know others," she says, "and you laugh further when you are fond of them or care for 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 reaction to it.

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

The Search for the Perfect Cracker Joke

Will we ever discover the perfect gag?

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

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

Over 40,000 jokes later, with ratings lodged by 350,000 people around the world, he has a better idea than many as to what succeeds and what does not.

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

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

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

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

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

"That's a common experience at the table and I think it's lovely."