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

This one-liner is greeted with groans that echo through a storage facility in London.

This describes a joke-testing session with a firm that produces supplies for gatherings. Its catalogue includes Christmas crackers.

The company's founder smiles, almost apologetically at the joke. But the pun has been selected and will feature in upcoming crackers.

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

The key to a great Christmas cracker pun is not the identical as a good joke in itself. It is all about the setting - in this instance, the communal amusement of the holiday meal with grandparents, children and possibly friends.

"You want the gag to be something that unites the child together with the grandparent," she adds.

The Neuroscience Of Communal Amusement

Coming together to experience communal laughter is not only ancient, experts argue, it is probably to be older than humanity.

"Therefore when you are laughing with others around the Christmas table you are engaging in what's very likely a really primordial mammal play sound," says a professor.

Shared amusement, she explains, helps forge and strengthen social bonds between individuals.

Researchers have discovered that a lack of such interactions can seriously harm both psychological and bodily health.

"The people you talk to, and share laughter with, it results in increased levels of endorphin uptake," the professor adds.

Endorphins are the brain's "happy chemicals" and are released both to alleviate stress and pain and in response to pleasurable activities, such as chuckling with loved ones over a truly awful festive cracker joke.

"It's not simply laughing at a foolish pun with a holiday cracker," she states. "You are in fact doing a lot of the really important task of building, preserving the connections you have with those you love."

What Occurs Inside the Brain?

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

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

Employing brain scanning technology, a type of brain scanner which indicates which areas of the brain are working harder, researchers have been able to chart the areas that get more blood flow.

The research involves scanning the brains of healthy subjects and then subjecting them to a database of funny phrases, accompanied by either a non-emotional sound, or recorded laughter.

"In the scanner we got a very fascinating pattern of activation," says the professor.

A gag stimulates not just the parts of the brain in charge of auditory processing and understanding speech, but also neural areas associated with both planning and starting movement and those involved in sight and memory.

Put these elements as a whole, and individuals hearing a joke have a complex set of brain responses that underpin the amusement we hear.

The Infectious Power of Laughter

Researchers found that when a humorous phrase is combined with chuckles there is a stronger response in the mind than the identical word when accompanied by a neutral sound.

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

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

Amusement, according to the expert, can be contagious.

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

"People laugh more when you know others," she says, "and you laugh more when you like them or love them."

When it comes to festive cracker puns, she explains, the positive effect is more likely to be caused not by the gag in itself, but from the reaction to it.

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

The Quest for the Perfect Festive Pun

Will we ever find the perfect gag?

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

In 2001, a psychologist established a research search for the world's funniest gag.

More than tens of thousands of jokes submitted, with ratings provided by hundreds of thousands of participants around the world, he has a clearer understanding than most as to what works and what fails.

The ideal festive cracker pun needs to be short, he explains.

"But they also be poor jokes, jokes that cause us to moan," he adds.

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

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

"What's interesting about the Christmas cracker puns is that not one person find them funny.

"It creates a shared moment around the table and I believe it's wonderful."