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

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

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

The company's owner grins, almost apologetically at the gag. But the joke has been selected and will appear in upcoming crackers.

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

The secret to a great Christmas cracker pun is not the identical as a good gag per se. It is entirely about the setting - in this instance, the communal amusement of the Christmas dinner table with grandparents, children and potentially friends.

"The goal is for the gag to be something that unites the child together with the 80-year-old," she states.

The Neuroscience Behind Communal Amusement

Gathering to enjoy communal amusement is not only ancient, scientists argue, it is probably to be pre-human.

"Therefore when you are laughing with people at the holiday table you are engaging in what's almost certainly a really ancient mammalian play sound," says a neuroscience expert.

Shared amusement, she explains, helps forge and strengthen social connections between individuals.

Scientists have found that a absence of these interactions can seriously harm mental and physical well-being.

"The people you converse with, and laugh with, it results in enhanced levels of 'happy chemical' release," the professor continues.

These natural chemicals are the body's "happy chemicals" and are produced both to reduce tension and discomfort and in reaction to enjoyable experiences, such as laughing with friends over a truly terrible festive cracker gag.

"It's not simply chuckling at a foolish joke with a Christmas cracker," she says. "You are in fact performing a lot of the really important work of making, maintaining the connections you have with the people you care about."

Which Happens In the Brain?

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

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

Employing functional magnetic resonance imaging (fMRI), a type of neural imager which indicates which parts of the brain are more active, scientists have been able to map the regions that get more blood.

Testing involves imaging the brains of volunteer subjects and then subjecting them to a collection of humorous phrases, paired with either a neutral sound, or recorded laughter.

"During the study we observed a very interesting pattern of activation," says the neuroscientist.

A joke activates not just the areas of the brain in charge of hearing and understanding speech, but also brain regions associated with both preparation and starting movement and those linked to sight and recall.

Put these elements as a whole, and people listening to a joke have a complex series of neural reactions that underpin the amusement we hear.

The Contagious Power of Chuckles

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

"This was in parts of the brain that you would employ to contort your expression into a grin or a laugh," the professor explains.

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

Amusement, says the expert, can be contagious.

So what does this mean for the laughter heard around a holiday table?

"You laugh harder when you are familiar with others," she notes, "and laughter increases further when you like them or care for them."

When it comes to Christmas cracker puns, she says, the feel-good effect is more probable to be caused not by the joke itself, but from the reaction to it.

"The laughter is key. The joke is the dreadful Christmas cracker pun, and it's just a reason to chuckle as a group."

The Quest for the Perfect Cracker Joke

Will we ever find the ultimate gag?

Likely not, but that has not stopped researchers from trying to.

Years ago, a professor established a scientific project for the planet's most humorous joke.

More than 40,000 gags submitted, with ratings lodged by 350,000 people globally, he has a better understanding than many as to what succeeds and what does not.

The perfect Christmas cracker joke needs to be brief, he explains.

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

The increasingly "awful" the joke, he states the better.

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

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

"That's a shared experience at the table and I believe it's lovely."