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

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

We're at a joke-testing session with a company that makes supplies for social events. Its repertoire features Christmas crackers.

The firm's owner smiles, 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 Christmas cracker pun is not the identical as a good gag per se. It is entirely about the context - in this case, the communal laughter of the Christmas dinner table with grandparents, children and potentially friends.

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

The Science Of Shared Amusement

Gathering to enjoy communal laughter is not only nothing new, scientists argue, it is likely to be older than humanity.

"Therefore when you are chuckling with others around the holiday dinner you are dropping into what's almost certainly a really ancient mammal play sound," explains a neuroscience expert.

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

Researchers have discovered that a absence of such interactions can seriously damage mental and physical well-being.

"Those you converse with, and share laughter with, it results in increased levels of 'happy chemical' uptake," the professor adds.

Endorphins are the brain's "feel-good compounds" and are released both to reduce tension and discomfort and in reaction to enjoyable experiences, such as laughing with friends over a particularly terrible Christmas cracker gag.

"You're not just chuckling at a silly pun with a holiday cracker," she says. "You are in fact doing a lot of the really important work of making, maintaining the connections you have with the people you love."

Which Occurs Inside the Brain?

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

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

Employing brain scanning technology, a kind of neural imager which shows which areas of the mind are working harder, scientists have been able to map the areas that receive more blood flow.

The research involves scanning the minds of healthy participants and then exposing them to a database of funny words, paired with either a non-emotional sound, or pre-recorded laughter.

"In the scanner we observed a really fascinating activation pattern of activation," says the professor.

A joke activates not just the parts of the mind responsible for auditory processing and understanding speech, but also neural regions associated with both preparation and starting movement and those involved in sight and recall.

Put all of this together, and individuals listening to a pun have a sophisticated set of brain responses that support the amusement we experience.

The Contagious Nature of Chuckles

Scientists discovered that when a humorous word is combined with laughter there is a stronger response in the mind than the same phrase when followed by a non-emotional sound.

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

It indicates people are not just reacting to humorous jokes, they are reacting to the amusement that follows them.

Amusement, says the professor, can be infectious.

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

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

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

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

The Search for the Perfect Cracker Joke

Will we ever find the ultimate joke?

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

Years ago, a professor set up a research search for the world's most humorous gag.

Over tens of thousands of jokes later, with scores lodged by 350,000 participants globally, he has a better idea than many as to what succeeds and what does not.

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

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

The more "awful" the gag, he says the better.

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

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

"That's a common moment at the gathering and I think it's lovely."