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

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

We're at a joke-testing session with a firm that makes supplies for social events. Its catalogue features festive crackers.

The firm's owner smiles, almost sheepishly at the gag. But the pun has been selected and will feature in future crackers.

"You measure the joke by the volume of moans and the loudness of the groans around the table," she says.

The secret to a great Christmas cracker joke is not the identical as a stand-up gag per se. It is all about the setting - in this instance, the shared amusement of the Christmas meal with elders, children and possibly friends.

"You want the gag to be a thing that brings the child together with the grandparent," she adds.

The Science Of Communal Laughter

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

"Therefore when you are chuckling with others around the Christmas table you are dropping into what's very likely a really ancient mammalian play sound," says a professor.

Shared amusement, she says, helps make and maintain social connections between individuals.

Researchers have found that a absence of such interactions can significantly harm both psychological and bodily well-being.

"The people you talk to, and share laughter with, it results in enhanced levels of 'happy chemical' release," the professor adds.

Endorphins are the brain's "feel-good compounds" and are released both to reduce tension and discomfort and in response to pleasurable activities, such as chuckling with loved ones over a particularly awful festive cracker gag.

"You're not just chuckling at a foolish joke with a holiday cracker," she states. "You are actually performing a lot of the really important task of building, preserving the connections you have with those you love."

What Happens Inside the Brain?

But what is truly taking place inside the brain when we hear a joke?

An awful lot occurs in response to comedy, it turns out.

Using brain scanning technology, a type of brain scanner which indicates which areas of the brain are working harder, researchers have been able to map the regions that get more blood.

Testing involves scanning the minds of volunteer participants and then exposing them to a collection of humorous words, paired with either a neutral sound, or recorded laughter.

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

A gag stimulates not just the parts of the brain responsible for auditory processing and understanding language, but also neural regions involved in both preparation and starting motion and those linked to sight and memory.

Combine these elements as a whole, and individuals listening to a pun have a complex set of neural reactions that underpin the amusement we hear.

The Infectious Nature of Laughter

Scientists discovered that when a humorous word is paired with chuckles there is a stronger reaction in the mind than the identical 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 smile or a laugh," the professor says.

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

Laughter, according to the professor, can be contagious.

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

"You laugh more when you know people," she notes, "and laughter increases further when you are fond of them or care for them."

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

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

The Quest for the Perfect Festive Pun

Is it possible to find the ultimate joke?

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

In 2001, a psychologist established a research project for the world's most humorous gag.

More than tens of thousands of jokes submitted, with ratings provided by 350,000 people around the world, he has a clearer understanding than most as to what works and what does not.

The ideal festive cracker joke must be short, he explains.

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

The increasingly "awful" the joke, he says the more effective.

"The reason is that if nobody laughs – it's the joke's fault, not your own.

"What's interesting about the Christmas cracker puns is that not one person find them funny.

"It creates a shared experience at the table and I believe it's wonderful."