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

This one-liner is met by groans that resonate through a warehouse in London.

We're at a joke-testing session with a company that produces supplies for gatherings. Its repertoire features Christmas crackers.

The firm's founder smiles, almost sheepishly at the joke. But the joke has been selected and will appear in future crackers.

"The success is gauged by the joke by the volume of groans and the intensity of the groans around the table," the founder says.

The key to a good Christmas cracker pun is not the identical as a stand-up joke per se. It is entirely about the context - in this case, the shared amusement of the Christmas dinner table with grandparents, kids and potentially neighbours.

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

The Science Behind Shared Amusement

Coming together to experience shared amusement is not only ancient, scientists say, it is likely to be pre-human.

"Therefore when you are chuckling with others around the holiday table you are engaging in what's almost certainly a truly ancient mammalian social vocalisation," explains a neuroscience expert.

Shared amusement, she says, aids in forge and strengthen social bonds between people.

Researchers have discovered that a absence of these social exchanges can seriously damage both psychological and bodily health.

"The people you converse with, and laugh with, it leads to enhanced levels of endorphin uptake," the professor adds.

Endorphins are the body's "feel-good compounds" and are produced both to reduce stress and pain and in response to pleasurable activities, such as chuckling with loved ones over a truly awful festive cracker joke.

"You're not just chuckling at a silly pun with a holiday cracker," the expert says. "You are in fact doing a lot of the really important task of building, preserving the connections you have with the people you love."

Which Happens Inside the Mind?

But what is actually taking place inside the brain when we listen to a gag?

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

Employing brain scanning technology, a kind of neural imager which shows which parts of the mind are working harder, scientists have been able to chart the regions that get more blood flow.

Testing entails scanning the brains of volunteer participants and then exposing them to a database of humorous words, paired with either a neutral sound, or recorded chuckles.

"In the scanner we got a very fascinating pattern of activation," says the neuroscientist.

A gag stimulates not just the parts of the brain responsible for hearing and interpreting language, but also brain areas associated with both planning and starting motion and those involved in sight and memory.

Combine these elements together, and people listening to a pun have a complex set of brain responses that support the laughter we hear.

The Contagious Nature of Chuckles

Scientists discovered that when a funny phrase is combined with laughter there is a greater response in the brain than the same word when followed by a neutral sound.

"This activation occurred in parts of the brain that you would use to move your expression into a grin or a laugh," the professor explains.

It means people are not just responding to funny jokes, they are responding to the laughter that accompanies them.

Amusement, according to the professor, can be infectious.

So what does this mean for the chuckles heard at a holiday table?

"You laugh more when you know people," she says, "and you laugh more when you like them or love them."

When it comes to festive cracker puns, she says, the positive factor is more likely to be triggered not by the joke in itself, but from the response to it.

"It's the laughter. The gag is the terrible Christmas cracker pun, and it's just a reason to laugh as a group."

The Search for the Perfect Cracker Joke

Will we ever discover the perfect gag?

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

In 2001, a psychologist established a scientific search for the planet's most humorous gag.

More than 40,000 jokes later, with ratings lodged by hundreds of thousands of people around the world, he has a better understanding than most as to what works and what does not.

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

"They must also be poor jokes, jokes that cause us to moan," he adds.

The more "awful" the gag, he states the better.

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

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

"That's a common experience around the gathering and I believe it's lovely."