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

This quip is met by groans that echo through a storage facility in London.

This describes a humor-evaluation meeting with a company that produces products for gatherings. Its repertoire features Christmas crackers.

The company's founder grins, nearly sheepishly at the gag. But the joke has made the cut and will feature in future crackers.

"The success is gauged by the gag by the number of moans and the intensity of the groans at the table," the founder says.

The key to a great Christmas cracker pun is not the identical as a good gag in itself. It is all about the context - in this case, the communal amusement of the Christmas meal with grandparents, kids and possibly neighbours.

"The goal is for the gag to be a thing that unites the eight-year-old together with the 80-year-old," she adds.

The Science Behind Shared Laughter

Gathering to experience shared amusement is not only nothing new, scientists say, it is probably to be pre-human.

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

Shared laughter, she says, aids in forge and strengthen social bonds between people.

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

"Those you talk to, and laugh with, it leads to increased amounts of 'happy chemical' release," the professor adds.

These natural chemicals are the brain's "feel-good compounds" and are produced both to alleviate stress and pain and in response to pleasurable experiences, such as laughing with friends over a particularly terrible Christmas cracker gag.

"You're not just chuckling at a foolish joke with a Christmas cracker," she states. "You are actually performing a lot of the truly vital task of making, maintaining the connections you have with those you love."

What Happens Inside the Brain?

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

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

Using brain scanning technology, a kind of brain scanner which indicates which parts of the mind are more active, scientists have been able to chart the areas that get more blood flow.

Testing entails scanning the brains of healthy participants and then subjecting them to a collection of humorous phrases, paired with either a non-emotional sound, or recorded chuckles.

"During the study we got a very interesting pattern of neural activity," says the neuroscientist.

A gag stimulates not just the parts of the brain in charge of hearing and understanding language, but also neural regions associated with both planning and starting movement and those linked to vision and memory.

Combine all of this as a whole, and individuals hearing a pun have a complex set of neural reactions that underpin the laughter we experience.

The Infectious Nature of Laughter

Scientists discovered that when a funny phrase is paired with laughter there is a stronger response in the mind than the identical phrase when accompanied by a neutral sound.

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

It indicates people are not just reacting to funny words, they are responding to the amusement that accompanies them.

Amusement, says the expert, can be contagious.

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

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

When it comes to Christmas cracker puns, she says, the feel-good effect is more probable to be triggered not by the gag in itself, but from the response to it.

"It's the laughter. The joke is the terrible holiday cracker pun, and it's just a reason to chuckle together."

The Search for the Perfect Cracker Joke

Is it possible to find the ultimate joke?

Likely not, but that has not prevented researchers from attempting to.

In 2001, a psychologist set up a scientific search for the planet's funniest joke.

More than 40,000 gags later, with scores lodged by hundreds of thousands of participants globally, he has a better understanding than many as to what succeeds and what does not.

The perfect Christmas cracker joke needs to be short, he says.

"But they also be bad gags, puns that cause us to moan," he adds.

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

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

"What's interesting about the holiday cracker puns is that not one person find them funny.

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