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

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

We're at a humor-evaluation meeting with a company that produces supplies for gatherings. Its catalogue features Christmas crackers.

The firm's owner grins, nearly sheepishly at the gag. But the pun has been selected and will feature in future crackers.

"The success is gauged by the gag by the number of groans and the intensity of the groans around the table," she explains.

The key to a good Christmas cracker pun is not the same as a good gag in itself. It is all about the setting - in this instance, the shared amusement of the holiday meal with elders, children and potentially friends.

"You want the gag to be something that brings the eight-year-old together with the 80-year-old," she adds.

The Neuroscience Of Communal Laughter

Gathering to enjoy communal amusement is not only ancient, scientists argue, it is probably to be older than humanity.

"So when you are chuckling with people around the Christmas dinner you are dropping into what's very likely a really primordial mammal play sound," says a neuroscience expert.

Communal laughter, she says, aids in forge and strengthen social connections between individuals.

Researchers have discovered that a lack of these interactions can seriously harm both psychological and bodily health.

"Those you converse with, and share laughter with, it results in enhanced amounts of endorphin release," she adds.

These natural chemicals are the brain's "happy chemicals" and are released both to alleviate stress and pain and in reaction to enjoyable activities, such as laughing with friends over a particularly terrible festive cracker joke.

"You're not just chuckling at a silly pun with a holiday cracker," the expert says. "You are actually doing a lot of the truly important work of building, preserving the connections you have with those you love."

What Occurs In the Mind?

But what is actually taking place inside the mind when we hear a gag?

An awful lot occurs in response to comedy, it transpires.

Employing functional magnetic resonance imaging (fMRI), a kind of neural imager which indicates which areas of the brain are more active, scientists have been able to chart the areas that receive more blood.

The research involves scanning the brains of volunteer subjects and then subjecting them to a collection of funny words, paired with either a neutral sound, or pre-recorded laughter.

"During the study we got a really fascinating pattern of neural activity," notes the professor.

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

Combine all of this as a whole, and people hearing a pun have a sophisticated series of neural responses that underpin the laughter we experience.

The Infectious Power of Chuckles

Researchers discovered that when a funny phrase is paired with laughter there is a stronger reaction in the mind than the identical word when followed by a non-emotional sound.

"This was in parts of the brain that you would use to contort your expression into a grin or a chuckle," the professor explains.

It means people are not just responding to humorous jokes, they are responding to the amusement that accompanies them.

Laughter, according to the professor, can be infectious.

So what does this imply for the laughter found at a holiday gathering?

"People laugh harder when you know people," she says, "and you laugh further when you like them or care for them."

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

"The laughter is key. The gag is the dreadful 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 ultimate gag?

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

In 2001, a professor set up a research search for the planet's funniest gag.

Over 40,000 gags submitted, with ratings lodged by hundreds of thousands of people globally, he has a better understanding than most as to what succeeds and what fails.

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

"But they also need to be poor jokes, jokes that cause us to groan," he continues.

The more "terrible" the gag, he says the more effective.

"The reason is that if nobody laughs – it's the gag's fault, not yours.

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

"That's a shared experience at the gathering and I believe it's lovely."