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

This quip is met by moans that resonate through a storage facility in London.

We're at a humor-evaluation session with a firm that makes supplies for social events. Its repertoire features Christmas crackers.

The company's founder smiles, almost sheepishly at the joke. But the pun has made the cut and will feature in upcoming crackers.

"You measure the gag by the number of groans and the intensity of the groans around the table," she says.

The secret to a good holiday cracker pun is not the same as a good joke in itself. It is entirely about the context - in this instance, the shared amusement of the Christmas dinner table with grandparents, kids and potentially neighbours.

"The goal is for the joke to be something that unites the child in harmony with the grandparent," she adds.

The Science Of Shared Amusement

Gathering to enjoy communal laughter is not only ancient, scientists say, it is likely to be pre-human.

"So when you are chuckling with people at the Christmas dinner you are dropping into what's almost certainly a truly primordial mammal play sound," says a neuroscience expert.

Shared laughter, she says, helps forge and strengthen social bonds between individuals.

Scientists have found that a absence of such social exchanges can significantly harm both psychological and bodily well-being.

"The people you talk to, and share laughter with, it results in enhanced levels of 'happy chemical' uptake," she adds.

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

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

What Happens Inside the Brain?

But what is actually happening inside the brain when we listen to a joke?

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

Using functional magnetic resonance imaging (fMRI), a type of brain scanner which shows which areas of the brain are more active, scientists have been able to chart the areas that get more blood flow.

The research involves imaging the brains of volunteer participants and then exposing them to a database of funny phrases, paired with either a neutral sound, or pre-recorded laughter.

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

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

Combine all of this together, and people listening to a joke have a complex series of brain reactions that support the amusement we hear.

The Contagious Nature of Chuckles

Scientists discovered that when a humorous word is combined with laughter there is a stronger response in the mind than the identical phrase when accompanied by a non-emotional sound.

"This was in areas of the brain that you would employ to contort your expression into a grin or a laugh," she explains.

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

Amusement, according to the expert, can be contagious.

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

"People laugh harder when you are familiar with others," she says, "and you laugh further when you like them or care for them."

When it comes to festive cracker jokes, she says, the positive factor is more probable to be triggered not by the joke itself, but from the response to it.

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

The Search for the Perfect Festive Pun

Will we ever discover the ultimate joke?

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

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

Over 40,000 jokes submitted, with scores provided by 350,000 participants around the world, he has a better idea than most as to what works and what does not.

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

"But they also be bad gags, jokes that make us groan," he adds.

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

"This is because if nobody finds it funny – it's the joke's fault, not your own.

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

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