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"What was the price did Father Christmas's sleigh cost? Nothing, it was on the house."

This one-liner is met by groans that echo through a warehouse in London.

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

The firm's founder smiles, nearly sheepishly at the gag. But the pun has been selected and will appear in future crackers.

"You measure the joke by the number of moans and the loudness of the groans around the table," she says.

The key to a great holiday cracker pun is not the same as a stand-up joke per se. It is entirely about the context - in this instance, the communal laughter of the Christmas meal with elders, kids and possibly neighbours.

"The goal is for the joke to be something that brings the eight-year-old in harmony with the grandparent," she adds.

The Neuroscience Of Shared Laughter

Coming together to experience communal laughter is not only nothing new, scientists argue, it is probably to be older than humanity.

"So when you are chuckling with others around the Christmas table you are engaging in what's almost certainly a truly ancient mammal play sound," says a professor.

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

Researchers have found that a absence of these social exchanges can significantly damage both psychological and bodily well-being.

"Those you talk to, and laugh with, it leads to enhanced amounts of 'happy chemical' uptake," the professor continues.

These natural chemicals are the brain's "feel-good compounds" and are released both to alleviate tension and discomfort and in reaction to pleasurable activities, such as chuckling with loved ones over a truly awful Christmas cracker joke.

"It's not simply laughing at a silly pun with a holiday cracker," she states. "You are in fact performing a lot of the really vital task of making, maintaining the connections you have with those you love."

Which Happens In the Mind?

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

A tremendous amount happens in reaction to humour, it transpires.

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

The research entails scanning the brains of volunteer subjects and then subjecting them to a collection of humorous phrases, accompanied by either a neutral sound, or pre-recorded laughter.

"In the scanner we observed a very interesting activation pattern of activation," says the neuroscientist.

A gag stimulates not just the areas of the brain responsible for hearing and interpreting speech, but also neural areas involved in both planning and initiating movement and those involved in sight and memory.

Put all of this as a whole, and individuals hearing a joke have a sophisticated set of neural responses that underpin the laughter we experience.

The Contagious Power of Chuckles

Scientists found that when a humorous phrase is paired with laughter there is a greater response in the mind than the same word when accompanied by a neutral sound.

"This was in areas of the brain that you would use to move your face into a grin or a laugh," she explains.

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

Laughter, according to the expert, can be infectious.

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

"People laugh harder when you are familiar with people," she notes, "and you laugh further when you are fond of them or love them."

When it comes to Christmas cracker puns, she says, the positive factor is more probable to be caused not by the joke itself, but from the reaction to it.

"The laughter is key. The joke is the dreadful Christmas cracker pun, and it's just a pretext to chuckle as a group."

The Search for the Ideal Festive Pun

Will we ever find the ultimate joke?

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

In 2001, a psychologist established a scientific project for the world's funniest joke.

More than tens of thousands of gags 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 joke needs to be short, he explains.

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

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

"The reason is that if nobody laughs – it's the gag's shortcoming, not your own.

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

"That's a shared experience around the table and I think it's lovely."