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"How much did Santa's sled cost? Nothing, it was on the house."

This quip is greeted with moans that resonate through a warehouse in London.

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

The firm's owner smiles, nearly apologetically at the joke. But the joke has made the cut and will appear in future crackers.

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

The key to a good Christmas cracker pun is not the same as a stand-up joke per se. It is all about the context - in this case, the shared laughter of the holiday dinner table with grandparents, kids and possibly neighbours.

"You want the joke to be something that brings the child together with the grandparent," she adds.

The Neuroscience Behind Shared Amusement

Coming together to enjoy shared laughter is not only ancient, experts argue, it is likely to be pre-human.

"So when you are laughing with others around the holiday table you are dropping into what's very likely a really primordial mammalian play vocalisation," says a professor.

Shared laughter, she explains, aids in make and maintain social connections between people.

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

"Those you talk to, and laugh with, it results in enhanced amounts of 'happy chemical' uptake," she continues.

These natural chemicals are the body's "happy chemicals" and are produced both to alleviate tension and discomfort and in reaction to enjoyable experiences, such as chuckling with friends over a particularly terrible Christmas cracker joke.

"You're not just chuckling at a silly pun with a Christmas cracker," she says. "You are in fact performing a lot of the truly vital task of building, preserving the social bonds you have with those you love."

Which Occurs In the Brain?

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

A tremendous amount occurs in reaction to comedy, it turns out.

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

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

"In the scanner we got a very fascinating pattern of neural activity," says the professor.

A gag activates not just the areas of the brain in charge of hearing and understanding speech, but also neural regions associated with both preparation and initiating motion and those involved in sight and recall.

Combine all of this as a whole, and individuals listening to a joke have a complex set of brain responses that underpin the amusement we experience.

The Infectious Nature of Chuckles

Researchers discovered that when a funny word is combined with laughter there is a stronger response in the brain than the identical word when accompanied by a non-emotional sound.

"This was in areas of the mind that you would employ to contort your face into a grin or a chuckle," she explains.

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

Laughter, according to the expert, can be contagious.

So what does this mean for the chuckles found around a holiday gathering?

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

When it comes to Christmas cracker jokes, she says, the positive factor 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 holiday cracker joke, and it's just a reason to laugh as a group."

The Quest for the Ideal Festive Pun

Is it possible to discover the perfect joke?

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

Years ago, a professor set up a research search for the planet's most humorous gag.

Over 40,000 gags later, with scores provided by hundreds of thousands of participants globally, he has a better idea than most as to what works and what fails.

The ideal Christmas cracker joke needs to be short, he explains.

"They must also need to be bad jokes, puns that make us moan," he continues.

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

"This is because if no-one laughs – it's the joke's shortcoming, not your own.

"The fascinating part about the holiday cracker jokes is that none of us find them funny.

"That's a common moment around the table and I think it's wonderful."