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

This joke is greeted with groans that resonate through a warehouse in the capital.

This describes a humor-evaluation session with a firm that makes products for gatherings. Its repertoire features Christmas crackers.

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

"The success is gauged by the gag by the volume of moans and the intensity of the groans around the table," the founder explains.

The secret to a great holiday cracker joke is not the same as a good gag per se. It is entirely about the setting - in this case, the communal amusement of the holiday meal with elders, children and potentially friends.

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

The Science Of Shared Laughter

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

"So when you are chuckling with people at the Christmas table you are dropping into what's almost certainly a truly primordial mammal play sound," explains a neuroscience expert.

Shared laughter, she says, helps make and maintain social bonds between individuals.

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

"The people you talk to, and laugh with, it leads to increased amounts of 'happy chemical' uptake," the professor adds.

These natural chemicals are the brain's "feel-good compounds" and are released both to reduce stress and pain and in reaction to pleasurable experiences, such as chuckling with loved ones over a particularly terrible Christmas cracker gag.

"It's not simply laughing at a silly joke with a holiday cracker," the expert says. "You are in fact performing a lot of the truly important work of making, maintaining the social bonds you have with the people you love."

Which Happens Inside the Mind?

But what is truly happening inside the brain when we listen to a gag?

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

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

Testing entails imaging the minds of volunteer subjects and then exposing them to a collection of funny words, paired with either a non-emotional sound, or pre-recorded laughter.

"During the study we got a really fascinating activation pattern of neural activity," notes the professor.

A joke activates not just the parts of the brain responsible for hearing and understanding speech, but also brain regions associated with both preparation and starting motion and those involved in vision and recall.

Put these elements together, and individuals hearing a joke have a complex series of neural responses that underpin the laughter we hear.

The Infectious Power of Laughter

Scientists found that when a funny word is combined with laughter there is a greater response in the mind than the identical phrase when followed by a non-emotional sound.

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

It means people are not just responding to humorous words, they are reacting to the laughter that follows them.

Laughter, says the professor, can be contagious.

So what does this imply for the chuckles found around a Christmas table?

"You laugh harder when you are familiar with people," she says, "and you laugh further when you like them or care for them."

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

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

The Search for the Perfect Cracker Joke

Will we ever find the ultimate gag?

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

In 2001, a professor established a research project for the planet's funniest joke.

More than tens of thousands of jokes later, with scores lodged by 350,000 participants globally, he has a better idea than most as to what succeeds and what fails.

The ideal festive cracker joke needs to be short, he says.

"They must also be poor gags, puns that cause us to groan," he adds.

The more "awful" the joke, he says the better.

"The reason is that if nobody finds it funny – it's the joke's shortcoming, not yours.

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

"It creates a common moment around the gathering and I believe it's lovely."