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

This one-liner is met by moans that echo through a storage facility in London.

We're at a humor-evaluation meeting with a firm that makes supplies for social events. Its repertoire features Christmas crackers.

The company's owner smiles, almost sheepishly at the joke. But the pun has made the cut and will feature in upcoming crackers.

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

The secret to a good holiday cracker joke is not the identical as a good gag per se. It is all about the setting - in this case, the shared amusement of the Christmas dinner table with elders, children and possibly neighbours.

"You want the gag to be a thing that brings the eight-year-old together with the 80-year-old," she adds.

The Science Behind Shared Amusement

Coming together to experience communal amusement is not only nothing new, scientists say, it is likely to be pre-human.

"Therefore when you are chuckling with others at the holiday dinner you are dropping into what's almost certainly a truly ancient mammal social vocalisation," explains a professor.

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

Researchers have discovered that a lack of these interactions can seriously damage both psychological and bodily well-being.

"Those you converse with, and share laughter with, it results in increased amounts of endorphin uptake," she continues.

These natural chemicals are the body's "feel-good compounds" and are produced both to reduce tension and discomfort and in response to pleasurable experiences, such as chuckling with loved ones over a particularly terrible festive cracker joke.

"You're not just laughing at a foolish joke with a holiday cracker," she states. "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."

What Happens In the Mind?

But what is truly happening within the brain when we listen to a joke?

An awful lot happens in reaction to comedy, it transpires.

Using functional magnetic resonance imaging (fMRI), a kind of neural imager which shows which areas of the brain are working harder, scientists have been able to map the regions that receive more blood.

The research involves scanning the brains of healthy subjects and then subjecting them to a database of humorous words, paired with either a non-emotional sound, or recorded laughter.

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

A joke stimulates not just the areas of the brain responsible for auditory processing and understanding language, but also neural areas involved in both preparation and initiating movement and those linked to vision and recall.

Put all of this as a whole, and individuals hearing a pun have a sophisticated set of neural responses that underpin the amusement we experience.

The Infectious Power of Chuckles

Researchers found that when a funny phrase is combined with laughter there is a greater response in the brain than the identical word when followed by a neutral sound.

"This was in parts of the mind that you would use to move your expression into a grin or a laugh," the professor says.

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

Laughter, says the expert, can be contagious.

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

"You laugh harder when you know others," she says, "and laughter increases further when you like them or love them."

When it comes to Christmas cracker puns, she says, the feel-good factor is more probable to be triggered not by the gag in itself, but from the reaction to it.

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

The Quest for the Ideal Cracker Joke

Is it possible to find the ultimate joke?

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

In 2001, a professor set up a scientific project for the planet's funniest joke.

Over tens of thousands of gags submitted, with ratings provided by hundreds of thousands of people around the world, he has a clearer idea than many as to what succeeds and what fails.

The perfect Christmas cracker joke must be short, he explains.

"They must also be poor jokes, jokes that make us groan," he continues.

The more "terrible" the gag, he says the more effective.

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

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

"It creates a common moment around the gathering and I think it's wonderful."