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

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

This describes a humor-evaluation meeting with a company that makes products for social events. Its repertoire includes festive crackers.

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

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

The key to a good Christmas cracker joke is not the identical as a good gag per se. It is all about the context - in this instance, the shared amusement of the holiday meal with elders, children and potentially neighbours.

"The goal is for the gag to be something that unites the eight-year-old in harmony with the 80-year-old," she states.

The Science Behind Communal Amusement

Coming together to experience communal amusement is not only nothing new, experts say, it is likely to be pre-human.

"So when you are chuckling with people at the Christmas table you are dropping into what's almost certainly a truly primordial mammalian play vocalisation," explains a neuroscience expert.

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

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

"The people you talk to, and share laughter with, it leads to increased amounts of endorphin release," she adds.

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

"It's not simply laughing at a foolish pun with a holiday cracker," she states. "You are in fact performing a lot of the really vital work of making, maintaining the social bonds you have with the people you love."

Which Happens In the Brain?

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

An awful lot happens in reaction to comedy, it transpires.

Using functional magnetic resonance imaging (fMRI), a type of neural imager which shows which parts of the mind are more active, scientists have been able to map the regions that receive more blood flow.

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

"In the scanner we observed a really interesting pattern of activation," notes the neuroscientist.

A gag activates not just the areas of the brain responsible for hearing and understanding language, but also neural regions involved in both planning and initiating movement and those involved in vision and recall.

Put all of this as a whole, and people hearing a pun have a complex set of neural reactions that support the laughter we experience.

The Contagious Power of Laughter

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

"This activation occurred in parts of the mind that you would employ to move your expression into a grin or a chuckle," she says.

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

Amusement, according to the professor, can be contagious.

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

"People laugh harder when you are familiar with others," she notes, "and you laugh more when you like them or love them."

When it comes to Christmas cracker puns, she says, 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 joke is the terrible Christmas cracker joke, and it's just a pretext to laugh as a group."

The Search for the Ideal Cracker Joke

Will we ever discover the perfect joke?

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

Years ago, a professor established a scientific project for the world's funniest joke.

More than 40,000 gags submitted, with scores lodged by 350,000 people around the world, he has a better understanding than most as to what works and what does not.

The perfect Christmas cracker pun needs to be brief, he explains.

"They must also need to be poor gags, jokes that make us groan," he continues.

The more "terrible" the gag, he states the better.

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

"What's interesting about the holiday cracker jokes is that none of us find them humorous.

"That's a common moment around the table and I believe it's lovely."