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

This one-liner is met by groans that echo through a warehouse in London.

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

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

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

The key to a good Christmas cracker pun is not the identical as a good joke per se. It is all about the context - in this instance, the shared laughter of the Christmas dinner table with elders, kids and potentially neighbours.

"You want the joke to be a thing that unites the eight-year-old in harmony with the grandparent," she adds.

The Science Of Shared Amusement

Coming together to enjoy shared laughter is not only nothing new, scientists say, it is probably to be pre-human.

"So when you are laughing with people around the holiday dinner you are engaging in what's almost certainly a truly primordial mammal play vocalisation," says a professor.

Communal amusement, she explains, helps forge and strengthen social connections between people.

Scientists have found that a absence of such social exchanges can significantly damage mental and physical health.

"The people you converse with, and share laughter with, it leads to increased levels of endorphin release," she continues.

These natural chemicals are the body's "feel-good compounds" and are produced both to alleviate stress and pain and in reaction to enjoyable experiences, such as chuckling with friends over a truly awful Christmas cracker gag.

"You're not just laughing at a foolish pun with a Christmas cracker," she states. "You are in fact performing a lot of the really vital task of building, preserving the connections you have with the people you care about."

Which Happens In the Brain?

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

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

Using functional magnetic resonance imaging (fMRI), a kind of brain scanner which shows which parts of the brain are working harder, scientists have been able to chart the areas that receive more blood.

The research entails scanning the brains of volunteer participants and then exposing them to a database of humorous words, accompanied by either a non-emotional sound, or recorded laughter.

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

A gag stimulates not just the areas of the brain responsible for auditory processing and understanding language, but also brain areas associated with both planning and initiating movement and those involved in sight and memory.

Put these elements together, and people hearing a joke have a complex series of neural reactions that support the laughter we hear.

The Contagious Power of Laughter

Scientists discovered that when a funny word is combined 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 brain that you would employ to move your expression into a smile or a laugh," she explains.

It indicates we are not just reacting to humorous words, they are reacting to the laughter that accompanies them.

Laughter, according to the professor, can be infectious.

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

"You laugh harder when you are familiar with people," she notes, "and laughter increases more when you are fond of them or care for them."

When it comes to Christmas cracker puns, she says, the positive effect is more probable to be triggered not by the gag itself, but from the reaction to it.

"The laughter is key. The gag is the terrible holiday cracker joke, and it's just a pretext to laugh as a group."

The Search for the Perfect Cracker Joke

Will we ever discover the perfect joke?

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

Years ago, a professor set up a research project for the world's funniest joke.

More than tens of thousands of jokes submitted, with ratings provided by 350,000 participants around the world, he has a better understanding than most as to what works and what fails.

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

"They must also be poor jokes, puns that cause us to groan," he adds.

The increasingly "terrible" the joke, he says the more effective.

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

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

"It creates a shared moment at the table and I think it's lovely."