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

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

We're at a joke-testing session with a firm that makes supplies for gatherings. Its catalogue includes Christmas crackers.

The firm's founder grins, almost apologetically at the joke. But the joke has made the cut and will appear in future crackers.

"The success is gauged by the gag by the volume of groans and the loudness of the groans at the table," she explains.

The key to a great holiday cracker pun is not the identical as a stand-up joke in itself. It is entirely about the setting - in this case, the shared amusement of the Christmas dinner table with grandparents, kids and possibly friends.

"You want the gag to be something that brings the eight-year-old in harmony with the grandparent," she adds.

The Neuroscience Of Communal Amusement

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

"So when you are chuckling with others around the Christmas table you are engaging in what's almost certainly a really ancient mammalian play vocalisation," says a professor.

Communal laughter, she says, helps forge and strengthen social bonds between individuals.

Scientists have found that a absence of such interactions can seriously harm both psychological and bodily well-being.

"Those you talk to, and share laughter with, it leads to increased levels of 'happy chemical' release," the professor continues.

Endorphins are the body's "happy chemicals" and are produced both to reduce tension and discomfort and in response to enjoyable experiences, such as chuckling with loved ones over a particularly awful Christmas cracker joke.

"It's not simply chuckling at a foolish pun with a holiday cracker," she says. "You are actually doing a lot of the truly vital task of building, preserving the connections you have with those you care about."

Which Happens In the Mind?

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

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

Using brain scanning technology, a type of neural imager which shows which areas of the mind are more active, researchers have been able to chart the areas that get more blood.

The research involves scanning the minds of healthy participants and then exposing them to a collection of funny phrases, accompanied by either a neutral sound, or recorded chuckles.

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

A joke stimulates not just the areas of the mind in charge of auditory processing and understanding speech, but also neural areas involved in both preparation and initiating motion and those involved in sight and memory.

Combine all of this as a whole, and individuals listening to a joke have a complex set of brain responses that support the amusement we hear.

The Infectious Power of Chuckles

Researchers discovered that when a humorous phrase is combined with chuckles there is a greater response in the brain than the same word when followed by a neutral sound.

"This was in areas of the brain that you would employ to move your face into a smile or a laugh," she explains.

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

Laughter, according to the expert, can be contagious.

So what does this imply for the laughter found around a Christmas gathering?

"You laugh more when you know people," she notes, "and you laugh more when you are fond of them or love them."

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

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

The Quest for the Ideal Festive Pun

Is it possible to discover the ultimate joke?

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

Years ago, a professor established a research project for the world's most humorous gag.

More than tens of thousands of gags submitted, with scores lodged by hundreds of thousands of people around the world, he has a better understanding than many as to what succeeds and what does not.

The perfect Christmas cracker pun must be brief, he says.

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

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

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

"What's interesting about the holiday cracker jokes is that none of us considers them funny.

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