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

This joke is greeted with moans that resonate through a storage facility in London.

We're at a joke-testing meeting with a company that produces products for social events. Its repertoire includes festive crackers.

The company's founder smiles, nearly apologetically at the joke. But the pun has made the cut and will feature in future crackers.

"You measure the gag by the volume of moans and the intensity of the groans at the table," the founder says.

The secret to a great Christmas cracker joke is not the identical as a good gag per se. It is all about the context - in this case, the shared amusement of the holiday meal with grandparents, kids and possibly friends.

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

The Science Behind Shared Laughter

Coming together to enjoy shared amusement is not only nothing new, scientists argue, it is likely to be pre-human.

"So when you are laughing with people around the Christmas table you are engaging in what's almost certainly a really primordial mammalian play vocalisation," explains a neuroscience expert.

Shared laughter, she says, helps forge and strengthen social connections between people.

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

"Those you converse with, and share laughter with, it results in enhanced amounts of endorphin uptake," the professor continues.

Endorphins are the body's "feel-good compounds" and are released both to alleviate stress and pain and in reaction to enjoyable activities, such as chuckling with loved ones over a particularly terrible Christmas cracker joke.

"You're not just laughing at a silly joke with a Christmas cracker," she says. "You are in fact performing a lot of the truly important work of making, maintaining the connections you have with those you love."

Which Occurs In the Mind?

But what is actually happening within the brain when we hear a gag?

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

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

Testing involves imaging the minds of healthy subjects and then exposing them to a collection of humorous phrases, accompanied by either a non-emotional sound, or recorded chuckles.

"During the study we observed a really fascinating pattern of activation," notes the neuroscientist.

A joke activates not just the parts of the brain in charge of hearing and understanding language, but also brain areas associated with both planning and starting movement and those involved in sight and memory.

Put these elements together, and people hearing a pun have a sophisticated set of neural responses that support the amusement we hear.

The Infectious Nature of Laughter

Researchers found that when a humorous word is paired with chuckles there is a greater reaction in the brain than the same phrase when accompanied by a non-emotional sound.

"This activation occurred in areas of the brain that you would use to move your expression into a smile or a laugh," she explains.

It means we are not just responding to funny words, they are responding to the laughter that follows them.

Amusement, says the expert, can be contagious.

So what does this imply for the laughter found around a Christmas gathering?

"You laugh harder when you know others," she says, "and you laugh more when you like them or care for them."

When it comes to Christmas cracker puns, she explains, the feel-good factor is more probable to be caused not by the gag in itself, but from the response to it.

"It's the laughter. The joke is the dreadful holiday cracker joke, and it's just a reason to chuckle as a group."

The Search for the Ideal Festive Pun

Is it possible to find the ultimate joke?

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

Years ago, a psychologist established a scientific search for the world's most humorous joke.

More than 40,000 jokes later, with scores provided by hundreds of thousands of participants globally, he has a better idea than most as to what succeeds and what fails.

The perfect festive cracker joke must be short, he explains.

"They must also be bad jokes, jokes that make us groan," he continues.

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

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

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

"It creates a common moment around the gathering and I think it's wonderful."