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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 storage facility in London.

This describes a joke-testing session with a company that produces products for social events. Its repertoire includes festive crackers.

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

"You measure the gag by the volume of groans and the intensity of the groans around the table," the founder explains.

The key to a great Christmas cracker pun is not the same as a good joke in itself. It is all about the context - in this case, the communal amusement of the holiday meal with elders, kids and potentially neighbours.

"The goal is for the joke to be a thing that brings the eight-year-old together with the grandparent," she states.

The Science Behind Communal Laughter

Gathering to experience shared amusement is not only nothing new, experts say, it is likely to be older than humanity.

"Therefore when you are chuckling with people around the holiday dinner you are engaging in what's almost certainly a really primordial mammal social vocalisation," explains a professor.

Shared laughter, she explains, aids in make and maintain social connections between people.

Scientists have found that a lack of these social exchanges can significantly harm mental and physical health.

"Those you talk to, and share laughter with, it leads to increased levels of endorphin release," she adds.

Endorphins are the brain's "feel-good compounds" and are released both to reduce stress and pain and in reaction to pleasurable activities, such as laughing with loved ones over a truly terrible festive cracker gag.

"It's not simply chuckling at a silly pun with a holiday cracker," she says. "You are in fact doing a lot of the really important work of building, preserving the connections you have with the people you love."

Which Happens Inside the Brain?

But what is truly happening within the mind when we hear a joke?

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

Employing brain scanning technology, a kind of neural imager which shows which areas of the brain are more active, researchers have been able to chart the regions that get more blood.

Testing entails scanning the minds of volunteer subjects and then exposing them to a database of humorous phrases, accompanied by either a neutral sound, or pre-recorded laughter.

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

A joke activates not just the areas of the brain responsible for auditory processing and understanding language, but also brain regions associated with both planning and initiating motion and those linked to sight and recall.

Combine all of this together, and individuals listening to a joke have a sophisticated set of neural reactions that underpin the laughter we experience.

The Contagious Power of Laughter

Scientists discovered that when a funny phrase is paired with chuckles there is a stronger response in the brain than the identical word when followed by a neutral sound.

"This was in parts of the brain that you would employ to move your face into a smile or a chuckle," she explains.

It means people are not just reacting to humorous words, they are reacting to the amusement that accompanies them.

Laughter, says the expert, can be infectious.

So what does this imply for the chuckles heard around a Christmas table?

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

When it comes to festive cracker puns, she explains, the feel-good factor is more likely to be caused not by the joke itself, but from the response to it.

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

The Quest for the Perfect Festive Pun

Is it possible to discover the ultimate joke?

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

Years ago, a psychologist set up a research project for the world's most humorous joke.

More than tens of thousands of jokes submitted, with scores lodged by hundreds of thousands of people around the world, he has a clearer idea than most as to what works and what fails.

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

"But they also need to be bad jokes, puns that make us moan," he adds.

The more "terrible" the gag, he says the more effective.

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

"The fascinating part about the Christmas cracker jokes is that none of us find them humorous.

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