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"How much did Santa's sleigh cost? Nothing, it was on the house."

This quip is met by groans that echo through a warehouse in London.

We're at a joke-testing meeting with a firm that makes products for gatherings. Its repertoire includes Christmas crackers.

The firm's founder smiles, nearly sheepishly at the gag. But the joke has made the cut and will feature in upcoming crackers.

"You measure the gag by the number of groans and the loudness of the groans at the table," she explains.

The secret to a great holiday cracker joke is not the same as a good gag in itself. It is entirely about the context - in this instance, the communal laughter of the Christmas dinner table with grandparents, children and possibly friends.

"You want the joke to be something that unites the eight-year-old together with the 80-year-old," she states.

The Science Of Shared Laughter

Coming together to enjoy communal amusement is not only nothing new, experts say, it is likely to be older than humanity.

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

Communal amusement, she says, aids in make and maintain social connections between individuals.

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

"Those you converse with, and laugh with, it results in enhanced amounts of 'happy chemical' uptake," she continues.

Endorphins are the body's "feel-good compounds" and are produced both to alleviate stress and pain and in response to enjoyable activities, such as chuckling with friends over a particularly awful festive cracker joke.

"You're not just laughing at a silly pun with a holiday cracker," the expert states. "You are in fact performing a lot of the truly vital work of building, preserving the social bonds you have with those you care about."

What Happens In the Brain?

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

A tremendous amount happens in response to comedy, it turns out.

Using brain scanning technology, a type of neural imager which indicates which parts of the mind are more active, scientists have been able to chart the regions that get more blood.

Testing entails scanning the brains of volunteer participants and then subjecting them to a collection of funny words, accompanied by either a neutral sound, or recorded chuckles.

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

A joke activates not just the areas of the mind responsible for auditory processing and interpreting language, but also neural regions involved in both planning and starting movement and those linked to sight and recall.

Combine these elements together, and people hearing a joke have a sophisticated series of brain responses that support the laughter we experience.

The Infectious Power of Laughter

Scientists found that when a humorous phrase is paired with chuckles there is a stronger reaction in the brain than the identical phrase when followed by a non-emotional sound.

"This activation occurred in parts of the mind that you would employ to contort your face into a smile or a laugh," she explains.

It indicates we are not just reacting to funny words, they are responding to the amusement that accompanies them.

Laughter, says the professor, can be infectious.

So what does this imply for the chuckles found at a Christmas table?

"People laugh more when you know people," she notes, "and you laugh further when you are fond of 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 joke itself, but from the reaction to it.

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

The Quest for the Perfect Festive Pun

Will we ever find the perfect gag?

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

In 2001, a psychologist set up a scientific project for the world's most humorous joke.

Over tens of thousands of jokes later, with ratings lodged by 350,000 people globally, he has a clearer idea than most as to what succeeds and what does not.

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

"But they also be bad gags, puns that cause us to groan," he continues.

The increasingly "awful" the gag, he says the more effective.

"The reason is that if no-one finds it funny – it's the gag's fault, not your own.

"What's interesting about the holiday cracker jokes is that not one person find them humorous.

"It creates a shared experience around the gathering and I believe it's wonderful."