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"How much did Santa's sled cost? Nothing, it was on the house."

This one-liner is greeted with moans that resonate through a warehouse in London.

We're at a humor-evaluation meeting with a firm that makes products for social events. Its catalogue features festive crackers.

The firm's founder smiles, nearly sheepishly at the joke. But the pun has been selected and will feature in future crackers.

"You measure the gag by the volume of moans and the intensity of the groans around the table," she explains.

The key to a great Christmas cracker pun is not the same as a good gag in itself. It is entirely about the context - in this instance, the communal laughter of the Christmas meal with elders, kids and possibly neighbours.

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

The Neuroscience Behind Shared Laughter

Gathering to experience communal laughter is not only ancient, experts argue, it is likely to be pre-human.

"Therefore when you are laughing with others around the holiday table you are dropping into what's almost certainly a truly primordial mammalian play sound," explains a professor.

Communal laughter, she explains, aids in make and maintain social bonds between individuals.

Scientists have found that a absence of such interactions can seriously damage both psychological and bodily health.

"Those you converse with, and laugh with, it leads to increased levels of endorphin release," she adds.

These natural chemicals are the body's "happy chemicals" and are released both to alleviate stress and pain and in reaction to pleasurable activities, such as laughing with loved ones over a truly awful festive cracker joke.

"It's not simply chuckling at a silly pun with a holiday cracker," she states. "You are actually doing a lot of the really important task of building, preserving the connections you have with those you love."

What Happens Inside the Mind?

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

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

Using brain scanning technology, a kind of neural imager which indicates which areas of the brain are more active, researchers have been able to map the areas that get more blood flow.

Testing entails scanning the minds of healthy participants and then subjecting them to a collection of funny phrases, accompanied by either a non-emotional sound, or pre-recorded laughter.

"During the study we observed a really fascinating activation pattern of neural activity," says the neuroscientist.

A gag activates not just the parts of the mind responsible for auditory processing and interpreting speech, but also brain areas involved in both preparation and starting movement and those involved in vision and recall.

Put all of this together, and individuals listening to a pun have a complex series of neural reactions that underpin the amusement we experience.

The Infectious Nature of Chuckles

Researchers discovered that when a humorous phrase is paired with chuckles there is a greater reaction in the brain than the same phrase when followed by a non-emotional sound.

"This was in areas of the brain that you would employ to move your expression into a smile or a chuckle," the professor says.

It indicates we are not just reacting to funny jokes, they are reacting to the amusement that accompanies them.

Amusement, according to the expert, can be infectious.

So what does this imply for the chuckles found at a Christmas gathering?

"You laugh more when you know others," she says, "and laughter increases more when you like them or love them."

When it comes to Christmas cracker jokes, she says, the positive factor is more likely to be triggered not by the joke itself, but from the response to it.

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

The Quest for the Perfect Festive Pun

Will we ever find the perfect gag?

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

Years ago, a professor established a scientific project for the planet's most humorous joke.

More than 40,000 jokes later, with ratings provided by hundreds of thousands of participants globally, he has a clearer understanding than most as to what works and what fails.

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

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

The increasingly "terrible" the gag, he states the better.

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

"The fascinating part about the holiday cracker puns is that none of us considers them humorous.

"It creates a common moment around the gathering and I believe it's wonderful."