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

This one-liner is greeted with moans that echo through a storage facility in London.

This describes a humor-evaluation meeting with a company that makes supplies for gatherings. Its repertoire features festive crackers.

The company's owner grins, nearly sheepishly at the gag. But the pun has been selected and will appear in future crackers.

"You measure the gag by the volume of groans and the loudness of the groans at the table," she explains.

The key to a good Christmas cracker joke is not the identical as a good joke in itself. It is all about the setting - in this case, the communal amusement of the Christmas meal with elders, children and possibly neighbours.

"You want the joke to be something that unites the child in harmony with the grandparent," she adds.

The Neuroscience Of Communal Amusement

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

"So when you are laughing with others around the Christmas dinner you are dropping into what's almost certainly a really primordial mammalian play vocalisation," says a neuroscience expert.

Shared laughter, she explains, aids in make and maintain social bonds between people.

Researchers have found that a absence of these interactions can seriously harm mental and physical health.

"The people you converse with, and laugh with, it results in increased amounts of 'happy chemical' release," she adds.

Endorphins are the brain's "feel-good compounds" and are released both to reduce tension and discomfort and in response to enjoyable experiences, such as laughing with loved ones over a particularly awful Christmas cracker joke.

"You're not just chuckling at a silly joke with a Christmas cracker," she states. "You are actually performing a lot of the really vital work of making, maintaining the social bonds you have with those you care about."

What Occurs In the Mind?

But what is truly happening inside the mind when we hear a gag?

A tremendous amount happens in response to comedy, it turns out.

Using brain scanning technology, a kind of neural imager which shows which areas of the mind are working harder, researchers have been able to map the areas that get more blood flow.

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

"During the study we observed a really interesting pattern of neural activity," says the professor.

A gag stimulates not just the areas of the mind responsible for auditory processing and understanding speech, but also brain areas involved in both planning and starting motion and those involved in sight and memory.

Put all of this as a whole, and individuals listening to a pun have a complex series of brain reactions that underpin the laughter we hear.

The Infectious Power of Laughter

Researchers found that when a funny word is paired with chuckles there is a greater reaction in the mind than the same phrase when accompanied by a non-emotional sound.

"This was in areas of the mind that you would employ to contort your expression into a grin or a chuckle," she says.

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

Amusement, according to the professor, can be infectious.

So what does this imply for the laughter heard at a Christmas gathering?

"People laugh more when you are familiar with people," she says, "and laughter increases more when you like them or care for them."

When it comes to festive cracker puns, 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 gag is the dreadful holiday cracker pun, and it's just a pretext to laugh as a group."

The Search for the Ideal Festive Pun

Is it possible to discover the perfect joke?

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

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

Over 40,000 gags submitted, with ratings lodged by 350,000 people around the world, he has a better idea than most as to what works and what fails.

The ideal Christmas cracker joke needs to be short, he explains.

"But they also need to be poor gags, puns that cause us to groan," he adds.

The increasingly "terrible" the joke, he says the better.

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

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

"That's a shared moment at the table and I think it's wonderful."