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

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

This describes a joke-testing meeting with a firm that makes supplies for gatherings. Its catalogue includes Christmas crackers.

The company's founder smiles, almost sheepishly at the joke. But the pun has been selected and will appear in future crackers.

"The success is gauged by the joke 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 same as a good joke in itself. It is all about the setting - in this case, the shared laughter of the Christmas dinner table with elders, kids and potentially neighbours.

"You want the gag to be a thing that brings the child in harmony with the 80-year-old," she adds.

The Neuroscience Of Shared Laughter

Coming together to enjoy shared laughter is not only nothing new, experts say, it is likely to be pre-human.

"Therefore when you are chuckling with people around the holiday table you are engaging in what's very likely a truly primordial mammalian play sound," explains a professor.

Communal amusement, she explains, aids in forge and strengthen social connections between people.

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

"Those you talk to, and laugh with, it results in enhanced levels of endorphin release," she continues.

Endorphins are the body's "happy chemicals" and are produced both to alleviate tension and discomfort and in reaction to pleasurable activities, such as laughing with friends over a truly terrible festive cracker joke.

"It's not simply laughing at a foolish joke with a Christmas cracker," she states. "You are actually performing a lot of the truly vital task of building, preserving the connections you have with those you love."

Which Occurs Inside the Mind?

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

An awful lot happens in response to humour, it turns out.

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

The research involves imaging the brains of healthy participants and then exposing them to a collection of humorous phrases, paired with either a neutral sound, or pre-recorded chuckles.

"In the scanner we got a very fascinating activation pattern of activation," notes the professor.

A joke stimulates not just the parts of the mind in charge of auditory processing and understanding speech, but also brain regions associated with both planning and initiating movement and those linked to sight and memory.

Put all of this together, and people listening to a joke have a sophisticated set of neural responses that support the laughter we hear.

The Infectious Power of Laughter

Scientists discovered that when a humorous phrase is paired with chuckles there is a stronger reaction in the brain than the same word when accompanied by a neutral sound.

"This activation occurred in areas of the mind that you would employ to contort your expression into a grin or a laugh," she explains.

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

Amusement, says the professor, can be infectious.

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

"People laugh more when you are familiar with people," she notes, "and laughter increases more when you are fond of them or love them."

When it comes to festive cracker puns, she says, the feel-good effect is more likely to be triggered not by the gag in itself, but from the response to it.

"It's the laughter. The joke is the terrible holiday cracker joke, and it's just a pretext to laugh together."

The Quest for the Ideal Festive Pun

Is it possible to find the ultimate gag?

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

In 2001, a professor established a scientific project for the world's most humorous gag.

More than 40,000 jokes submitted, with scores provided by hundreds of thousands of people around the world, he has a better idea than many as to what succeeds and what does not.

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

"They must also be bad gags, jokes that make us moan," he adds.

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

"This is because if nobody finds it funny – it's the joke's shortcoming, not yours.

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

"That's a common moment around the table and I think it's lovely."