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"What was the price did Father Christmas's sled cost? Nothing, it was on the house."

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

This describes a humor-evaluation meeting with a firm that produces products for social events. Its catalogue includes Christmas crackers.

The firm's founder smiles, almost apologetically at the gag. But the joke has been selected and will appear in future crackers.

"You measure the joke by the volume of groans and the loudness of the groans around the table," the founder explains.

The key to a great Christmas cracker joke is not the same as a good joke in itself. It is entirely about the setting - in this case, the communal laughter of the holiday meal with grandparents, children and potentially friends.

"The goal is for the joke to be something that brings the child together with the grandparent," she adds.

The Science Behind Shared Laughter

Gathering to enjoy shared amusement is not only ancient, experts say, it is probably to be older than humanity.

"So when you are laughing with others at the Christmas dinner you are dropping into what's very likely a truly ancient mammal play vocalisation," says a professor.

Shared amusement, she says, helps make and maintain social bonds between people.

Scientists have discovered that a absence of these interactions can seriously damage mental and physical health.

"The people you converse with, and share laughter with, it results in enhanced levels of endorphin release," the professor continues.

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

"It's not simply laughing at a foolish pun with a Christmas cracker," she states. "You are actually doing a lot of the really important work of building, preserving the social bonds you have with the people you care about."

Which Occurs Inside the Mind?

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

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

Using brain scanning technology, a kind of neural imager which indicates which parts of the brain are working harder, scientists have been able to map the regions that receive more blood flow.

The research involves scanning the minds of healthy participants and then subjecting them to a database of humorous phrases, accompanied by either a non-emotional sound, or recorded chuckles.

"During the study we got a really fascinating activation pattern of activation," notes the neuroscientist.

A gag stimulates not just the parts of the mind in charge of hearing and understanding language, but also brain areas associated with both preparation and starting motion and those involved in sight and memory.

Put all of this together, and individuals listening to a joke have a complex set of neural responses that support the laughter we experience.

The Contagious Nature of Laughter

Researchers found that when a humorous word is combined with laughter there is a stronger reaction in the mind than the identical phrase when accompanied by a non-emotional sound.

"This activation occurred in parts of the mind that you would use to move your face into a smile or a laugh," the professor explains.

It indicates we are not just reacting to humorous words, they are reacting to the laughter that follows them.

Laughter, says the expert, can be contagious.

So what does this mean for the chuckles found at a holiday gathering?

"People laugh more when you know others," she notes, "and you laugh further when you are fond of them or love them."

When it comes to festive cracker jokes, she says, the feel-good effect is more likely to be caused not by the joke itself, but from the response to it.

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

The Quest for the Ideal Festive Pun

Will we ever discover the ultimate joke?

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

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

Over tens of thousands of jokes later, with ratings lodged by hundreds of thousands of people globally, he has a better understanding than many as to what works and what does not.

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

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

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

"The reason is that if no-one laughs – it's the joke's shortcoming, not your own.

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

"That's a shared moment around the gathering and I believe it's wonderful."