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

This one-liner is greeted with groans that echo through a warehouse in the capital.

We're at a joke-testing session with a firm that makes products for social events. Its catalogue includes festive crackers.

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

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

The secret to a good Christmas cracker joke is not the same as a good gag in itself. It is all about the context - in this instance, the communal amusement of the holiday dinner table with grandparents, children and potentially friends.

"You want the joke to be something that brings the child together with the 80-year-old," she states.

The Science Behind Shared Amusement

Coming together to enjoy communal laughter is not only nothing new, experts say, it is likely to be older than humanity.

"Therefore when you are laughing with people at the Christmas dinner you are dropping into what's very likely a really ancient mammalian play vocalisation," explains a professor.

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

Scientists have discovered that a absence of such social exchanges can significantly harm both psychological and bodily well-being.

"Those you talk to, and laugh with, it leads to increased amounts of endorphin uptake," she adds.

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

"You're not just chuckling at a silly pun with a holiday cracker," the expert says. "You are in fact performing a lot of the really important task of making, maintaining the social bonds you have with the people you care about."

What Happens Inside the Mind?

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

An awful lot occurs in reaction to humour, it transpires.

Using brain scanning technology, a type of neural imager which indicates which parts of the mind are more active, scientists have been able to chart the regions that get more blood.

Testing involves scanning the brains of healthy subjects and then exposing them to a collection of humorous words, accompanied by either a neutral sound, or pre-recorded chuckles.

"In the scanner we observed a very interesting pattern of neural activity," notes the neuroscientist.

A gag activates not just the areas of the mind responsible for auditory processing and interpreting speech, but also neural areas associated with both planning and starting movement and those linked to vision and memory.

Put all of this together, and people hearing a joke have a sophisticated series of neural responses that support the laughter we experience.

The Contagious Nature of Chuckles

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

"This was in parts of the brain that you would employ to contort your face into a smile or a chuckle," she says.

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

Amusement, says the professor, can be contagious.

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

"You laugh harder when you are familiar with others," she says, "and you laugh further when you are fond of them or love them."

When it comes to festive cracker puns, she says, the feel-good effect is more probable to be caused not by the gag itself, but from the reaction to it.

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

The Quest for the Perfect Cracker Joke

Is it possible to discover the perfect joke?

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

Years ago, a psychologist set up a research search for the planet's funniest gag.

Over tens of thousands of gags submitted, with ratings provided by hundreds of thousands of participants around the world, he has a clearer understanding than most as to what succeeds and what does not.

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

"They must also be poor jokes, jokes that make us moan," he continues.

The more "awful" the joke, he says the more effective.

"This is because if nobody laughs – it's the gag's fault, not your own.

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

"It creates a common experience around the gathering and I think it's wonderful."