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

This quip is greeted with groans that resonate through a storage facility in London.

This describes a humor-evaluation session with a firm that makes supplies for social events. Its catalogue includes festive crackers.

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

"You measure the joke by the volume of groans and the loudness of the groans around the table," the founder explains.

The secret to a great Christmas cracker joke is not the same as a stand-up joke per se. It is all about the context - in this instance, the communal amusement of the Christmas meal with elders, children and possibly neighbours.

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

The Science Behind Communal Laughter

Gathering to experience shared laughter is not only ancient, experts say, it is probably to be pre-human.

"Therefore when you are chuckling with others around the holiday table you are engaging in what's almost certainly a really ancient mammalian play sound," explains a neuroscience expert.

Communal amusement, she says, aids in forge and strengthen social connections between people.

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

"The people you converse with, and laugh with, it results in increased levels of endorphin release," she continues.

These natural chemicals are the brain's "feel-good compounds" and are produced both to reduce stress and pain and in response to enjoyable activities, such as chuckling with loved ones over a particularly awful Christmas cracker gag.

"You're not just chuckling at a silly joke with a Christmas cracker," she says. "You are in fact doing a lot of the really important work of building, preserving the social bonds you have with those you care about."

Which Occurs In the Mind?

But what is truly taking place inside the brain when we hear a joke?

An awful lot happens in response to comedy, it turns out.

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

Testing involves scanning the brains of volunteer participants and then subjecting them to a database of funny words, paired with either a non-emotional sound, or pre-recorded laughter.

"During the study we got a very interesting pattern of activation," says the professor.

A joke activates not just the parts of the brain in charge of auditory processing and interpreting speech, but also brain regions involved in both preparation and initiating movement and those linked to sight and memory.

Put all of this together, and individuals hearing a joke have a sophisticated series of brain reactions that underpin the amusement we experience.

The Infectious Power of Laughter

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

"This activation occurred in areas of the mind that you would use to contort your expression into a smile or a chuckle," the professor says.

It means people are not just responding to funny jokes, they are reacting to the amusement that follows them.

Laughter, according to the professor, can be contagious.

So what does this mean for the laughter heard at a Christmas table?

"People laugh more when you are familiar with people," she notes, "and you laugh further when you are fond of them or care for them."

When it comes to festive cracker jokes, she explains, the feel-good effect is more likely to be caused not by the gag in itself, but from the reaction to it.

"It's the laughter. The gag is the terrible Christmas cracker pun, and it's just a pretext to laugh together."

The Quest for the Perfect Cracker Joke

Will we ever discover the ultimate joke?

Likely not, but that has not prevented experts from trying to.

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

More than 40,000 gags submitted, with ratings provided by 350,000 participants globally, he has a clearer understanding than many as to what works and what fails.

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

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

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

"This is because if no-one laughs – it's the gag's fault, not yours.

"The fascinating part about the Christmas cracker puns is that not one person considers them humorous.

"That's a common experience at the table and I think it's wonderful."