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

This quip is greeted with moans that echo through a warehouse in London.

We're at a joke-testing meeting with a company that produces supplies for gatherings. Its repertoire includes Christmas crackers.

The firm's owner smiles, nearly sheepishly at the joke. But the pun has made the cut and will appear in upcoming crackers.

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

The key to a great 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 holiday dinner table with elders, children and possibly neighbours.

"You want the joke to be a thing that brings the child in harmony with the grandparent," she adds.

The Science Behind Shared Laughter

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

"Therefore when you are laughing with people at the holiday dinner you are engaging in what's very likely a truly ancient mammal play sound," says a professor.

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

Scientists have discovered that a lack of such interactions can significantly damage mental and physical well-being.

"Those you converse with, and laugh with, it leads to increased amounts of 'happy chemical' uptake," the professor continues.

These natural chemicals are the body's "feel-good compounds" and are produced both to alleviate stress and pain and in response to enjoyable activities, such as laughing with friends over a particularly terrible Christmas cracker joke.

"You're not just laughing at a foolish joke with a Christmas cracker," she states. "You are actually performing a lot of the really vital task of building, preserving the social bonds you have with the people you care about."

What Occurs In the Mind?

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

A tremendous amount occurs in reaction to comedy, it transpires.

Using functional magnetic resonance imaging (fMRI), a type of brain scanner which indicates which areas of the mind are more active, scientists have been able to map the regions that get more blood.

Testing involves scanning the brains of volunteer participants and then exposing them to a database of humorous words, accompanied by either a neutral sound, or recorded chuckles.

"In the scanner we got a really interesting pattern of activation," notes the professor.

A joke activates not just the parts of the brain in charge of auditory processing and interpreting speech, but also brain areas associated with both preparation and initiating movement and those linked to vision and recall.

Put all of this as a whole, and people listening to a pun have a complex series of brain reactions that support the amusement we hear.

The Contagious Nature of Laughter

Scientists discovered that when a humorous phrase is paired with laughter there is a stronger response in the mind than the identical word when followed by a neutral sound.

"This activation occurred in areas of the mind that you would employ to contort your face into a smile or a laugh," the professor explains.

It means people are not just reacting to funny words, they are reacting to the amusement that follows them.

Amusement, according to the expert, can be infectious.

So what does this imply for the chuckles heard around a holiday gathering?

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

When it comes to Christmas cracker puns, she explains, the positive effect is more probable to be caused not by the gag in itself, but from the reaction to it.

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

The Search for the Ideal Festive Pun

Is it possible to discover the ultimate joke?

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

Years ago, a professor set up a scientific search for the planet's most humorous joke.

More than tens of thousands of jokes later, with scores lodged by 350,000 people around the world, he has a better idea than many as to what succeeds and what does not.

The ideal Christmas cracker joke must be brief, he says.

"They must also need to be poor jokes, jokes that cause us to groan," he adds.

The increasingly "awful" the gag, he says the more effective.

"This is because if no-one laughs – it's the gag's shortcoming, not your own.

"The fascinating part about the Christmas cracker jokes is that none of us find them humorous.

"That's a shared experience around the gathering and I think it's lovely."