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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 humor-evaluation meeting with a firm that makes supplies for social events. Its repertoire features festive crackers.

The company's founder grins, nearly sheepishly at the joke. But the joke has been selected and will feature in upcoming crackers.

"You measure the gag by the volume of moans and the loudness of the groans at the table," she explains.

The secret to a good Christmas cracker pun is not the same as a stand-up joke in itself. It is entirely about the setting - in this instance, the communal laughter of the holiday dinner table with grandparents, kids and possibly friends.

"The goal is for the joke to be something that brings the eight-year-old in harmony with the 80-year-old," she adds.

The Neuroscience Of Shared Laughter

Gathering to experience shared amusement is not only nothing new, scientists argue, it is probably to be pre-human.

"Therefore when you are laughing with people at the Christmas table you are engaging in what's very likely a really ancient mammal social vocalisation," explains a neuroscience expert.

Communal laughter, she explains, aids in forge and strengthen social connections between individuals.

Scientists have found that a lack of these social exchanges can seriously harm mental and physical well-being.

"Those you talk to, and share laughter with, it results in increased amounts of endorphin uptake," the professor continues.

These natural chemicals are the brain's "feel-good compounds" and are released both to alleviate stress and pain and in response to pleasurable activities, such as laughing with friends over a particularly awful Christmas cracker gag.

"You're not just laughing at a foolish joke with a holiday cracker," the expert says. "You are actually doing a lot of the truly important work of making, maintaining the social bonds you have with those you care about."

What Happens In the Mind?

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

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

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

Testing involves scanning the brains of volunteer participants and then subjecting them to a collection of humorous words, paired with either a neutral sound, or pre-recorded chuckles.

"In the scanner we got a really interesting pattern of activation," says the neuroscientist.

A joke activates not just the areas of the brain responsible for auditory processing and interpreting language, but also brain areas associated with both preparation and initiating movement and those involved in vision and memory.

Put these elements together, and people listening to a joke have a complex series of brain responses that support the amusement we hear.

The Infectious Power of Laughter

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

"This activation occurred in parts of the mind that you would employ to move your face into a smile or a chuckle," she says.

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

Laughter, according to the professor, can be contagious.

So what does this imply for the laughter found at a holiday gathering?

"You laugh more when you know others," she notes, "and laughter increases more when you like them or care for them."

When it comes to Christmas cracker puns, she explains, the positive factor is more likely to be caused not by the gag in itself, but from the reaction to it.

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

The Search for the Perfect Cracker Joke

Will we ever discover the ultimate joke?

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

In 2001, a professor established a scientific search for the world's funniest gag.

More than 40,000 jokes later, with scores lodged by hundreds of thousands of participants globally, he has a clearer idea than most as to what succeeds and what fails.

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

"But they also be bad gags, jokes that make us groan," he continues.

The increasingly "terrible" the joke, he says the better.

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

"What's interesting about the Christmas cracker jokes is that none of us considers them funny.

"That's a shared experience at the gathering and I think it's wonderful."