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

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

This describes a joke-testing session with a company that produces supplies for gatherings. Its catalogue features festive crackers.

The firm's owner grins, almost sheepishly at the gag. But the joke has made the cut and will feature in future crackers.

"You measure the joke by the number of moans and the intensity of the groans around the table," the founder explains.

The key to a good Christmas cracker joke is not the same as a good joke in itself. It is entirely about the context - in this case, the shared laughter of the Christmas meal with elders, kids and possibly neighbours.

"The goal is for the joke to be a thing that brings the child together with the 80-year-old," she states.

The Neuroscience Behind Communal Amusement

Coming together to enjoy communal laughter is not only ancient, experts argue, it is probably to be pre-human.

"So when you are chuckling with others around the Christmas table you are dropping into what's almost certainly a truly primordial mammalian play vocalisation," says a professor.

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

Researchers have found that a absence of such social exchanges can seriously damage both psychological and bodily well-being.

"Those you talk to, and share laughter with, it results in enhanced amounts of endorphin uptake," the professor adds.

These natural chemicals are the body's "feel-good compounds" and are released both to reduce tension and discomfort and in reaction to pleasurable activities, such as chuckling with friends over a truly terrible Christmas cracker gag.

"It's not simply laughing at a silly pun with a holiday cracker," she states. "You are actually performing a lot of the really important work of making, maintaining the social bonds you have with those you care about."

What Happens Inside the Brain?

But what is truly happening inside the mind when we listen to a joke?

A tremendous amount happens in reaction to comedy, it turns out.

Using functional magnetic resonance imaging (fMRI), a kind of brain scanner which shows which parts of the mind are more active, researchers have been able to map the regions that receive more blood flow.

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

"During the study we observed a very interesting pattern of neural activity," notes the professor.

A gag stimulates not just the parts of the mind in charge of auditory processing and understanding language, but also neural areas involved in both planning and starting motion and those linked to vision and memory.

Put these elements together, and individuals listening to a pun have a sophisticated series of brain responses that support the amusement we hear.

The Contagious Power of Laughter

Researchers discovered that when a funny phrase is paired with chuckles there is a stronger response in the brain than the same phrase when followed by a neutral sound.

"This was in areas of the mind that you would use to contort your face into a grin or a chuckle," the professor explains.

It indicates we are not just responding to humorous words, they are responding to the laughter that accompanies them.

Laughter, according to the expert, can be infectious.

So what does this imply for the chuckles found around a holiday table?

"You laugh more when you are familiar with people," she notes, "and laughter increases more when you are fond of them or love them."

When it comes to festive cracker puns, she explains, the positive factor is more probable to be triggered not by the gag in itself, but from the response to it.

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

The Search for the Perfect Festive Pun

Is it possible to find the ultimate joke?

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

In 2001, a psychologist set up a scientific project for the planet's most humorous joke.

More than tens of thousands of jokes submitted, with scores provided by hundreds of thousands of participants around the world, he has a clearer idea than most as to what succeeds and what does not.

The ideal festive cracker pun must be brief, he says.

"They must also need to be poor jokes, puns that cause us to moan," he continues.

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

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

"The fascinating part about the holiday cracker jokes is that not one person considers them funny.

"That's a shared experience at the gathering and I think it's wonderful."