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

This one-liner is met by moans that resonate through a warehouse in London.

This describes a humor-evaluation session with a firm that produces products for social events. Its repertoire includes festive crackers.

The firm's owner smiles, almost apologetically at the gag. But the pun has been selected and will feature in future crackers.

"The success is gauged by the joke by the volume of groans and the intensity of the groans around the table," the founder explains.

The key to a good holiday cracker joke is not the same as a stand-up gag per se. It is entirely about the context - in this instance, the shared amusement of the holiday meal with elders, children and possibly neighbours.

"You want the joke to be a thing that unites the child in harmony with the 80-year-old," she states.

The Neuroscience Of Shared Amusement

Coming together to experience communal laughter is not only ancient, experts say, it is likely to be pre-human.

"So when you are chuckling with people around the Christmas table you are engaging in what's very likely a truly primordial mammalian play vocalisation," says a neuroscience expert.

Shared laughter, she explains, helps make and maintain social bonds between people.

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

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

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 truly terrible festive cracker joke.

"You're not just laughing at a foolish joke with a Christmas cracker," the expert states. "You are in fact performing a lot of the truly important work of building, preserving the connections you have with the people you love."

What Occurs In the Mind?

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

A tremendous amount occurs in reaction to humour, it turns out.

Employing functional magnetic resonance imaging (fMRI), a type of neural imager which indicates which areas of the mind are working harder, researchers have been able to map the regions that get more blood.

The research entails imaging the brains of healthy participants and then exposing them to a database of funny words, accompanied by either a neutral sound, or pre-recorded chuckles.

"In the scanner we got a very interesting activation pattern of neural activity," notes the neuroscientist.

A joke activates not just the areas of the mind responsible for auditory processing and understanding speech, but also brain areas involved in both preparation and starting movement and those linked to sight and recall.

Put all of this as a whole, and people listening to a joke have a complex series of brain responses that underpin the laughter we experience.

The Contagious Power of Laughter

Scientists discovered that when a funny word is combined with laughter there is a greater response in the brain than the identical phrase when accompanied by a neutral sound.

"This was in parts of the mind that you would use to move your expression into a smile or a laugh," she says.

It means people are not just reacting to humorous words, they are responding to the laughter that follows them.

Laughter, according to the professor, can be infectious.

So what does this imply for the laughter heard at a Christmas gathering?

"People laugh harder when you know others," she says, "and laughter increases further when you like them or love them."

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

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

The Quest for the Perfect Cracker Joke

Will we ever discover the ultimate joke?

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

Years ago, a professor set up a research search for the planet's funniest joke.

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

The ideal Christmas cracker pun must be short, he explains.

"But they also be poor jokes, puns that make us moan," he continues.

The increasingly "awful" the joke, he states the more effective.

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

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

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