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

This joke is met by moans that echo through a storage facility in the capital.

We're at a humor-evaluation session with a company that makes products for social events. Its repertoire features festive crackers.

The company's owner grins, nearly apologetically at the joke. But the pun has been selected and will feature in future crackers.

"You measure the joke by the number of groans and the intensity of the groans around the table," she says.

The secret to a good Christmas cracker pun is not the identical as a good joke in itself. It is entirely about the setting - in this instance, the shared amusement of the holiday meal with elders, children and potentially friends.

"You want the gag to be a thing that brings the eight-year-old in harmony with the grandparent," she states.

The Neuroscience Behind Shared Amusement

Gathering to experience shared laughter is not only ancient, experts say, it is likely to be older than humanity.

"Therefore when you are chuckling with people around the holiday table you are engaging in what's almost certainly a really ancient mammal social vocalisation," says a professor.

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

Researchers have discovered that a lack of these interactions can seriously harm both psychological and bodily health.

"The people you converse with, and laugh with, it leads to increased amounts of endorphin uptake," the professor continues.

Endorphins are the body's "happy chemicals" and are produced both to alleviate stress and pain and in response to enjoyable experiences, such as laughing with friends over a truly terrible festive cracker gag.

"It's not simply laughing at a silly pun with a Christmas cracker," the expert states. "You are actually doing a lot of the really vital work of making, maintaining the connections you have with the people you care about."

Which Occurs In the Brain?

But what is actually happening within the brain when we listen to a gag?

An awful lot happens in reaction to comedy, it turns out.

Using brain scanning technology, a type of neural imager which shows which parts of the mind are more active, scientists have been able to chart 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 phrases, accompanied by either a neutral sound, or recorded chuckles.

"In the scanner we got a really interesting activation pattern of neural activity," notes the neuroscientist.

A gag activates not just the parts of the mind in charge of hearing and interpreting language, but also neural regions involved in both preparation and initiating movement and those involved in vision and memory.

Put these elements together, and individuals listening to a joke have a sophisticated series of neural reactions that support the amusement we hear.

The Infectious Nature of Chuckles

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

"This activation occurred in areas of the brain that you would employ to move your face into a smile or a chuckle," she says.

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

Amusement, says the professor, can be contagious.

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

"People laugh more when you are familiar with others," she says, "and you laugh further when you are fond of them or care for them."

When it comes to festive cracker jokes, she says, the positive factor is more probable to be caused not by the joke in itself, but from the response to it.

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

The Search for the Perfect Cracker Joke

Is it possible to find the ultimate gag?

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

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

More than tens of thousands of gags submitted, with scores lodged by 350,000 people globally, he has a better understanding than most as to what succeeds and what fails.

The ideal festive cracker pun must be short, he says.

"They must also need to be bad jokes, jokes that make us groan," he continues.

The increasingly "terrible" the gag, he states the more effective.

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

"What's interesting about the Christmas cracker jokes is that not one person considers them humorous.

"That's a common experience around the table and I think it's wonderful."