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

This joke is met by moans that resonate through a warehouse in London.

We're at a humor-evaluation session with a company that makes products for social events. Its repertoire features Christmas crackers.

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

"The success is gauged by the gag by the volume of moans and the loudness of the groans at the table," the founder says.

The key to a great holiday cracker pun is not the identical as a stand-up gag in itself. It is all about the context - in this case, the shared laughter of the Christmas meal with grandparents, children and possibly neighbours.

"The goal is for the joke to be a thing that unites the child in harmony with the 80-year-old," she adds.

The Neuroscience Behind Communal Laughter

Coming together to enjoy shared amusement is not only ancient, scientists say, it is probably to be older than humanity.

"So when you are laughing with people at the Christmas table you are dropping into what's almost certainly a really primordial mammal play vocalisation," explains a neuroscience expert.

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

Scientists have found that a absence of such interactions can seriously harm mental and physical health.

"Those you converse with, and share laughter with, it results in enhanced levels of endorphin release," the professor adds.

These natural chemicals are the brain's "happy chemicals" and are released both to alleviate stress and pain and in response to pleasurable activities, such as laughing with loved ones over a truly terrible festive cracker gag.

"It's not simply chuckling at a silly pun with a holiday cracker," she says. "You are actually performing a lot of the truly important task of building, preserving the social bonds you have with those you love."

What Happens In the Brain?

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

A tremendous amount happens in response to humour, it transpires.

Using brain scanning technology, a kind of brain scanner which indicates which parts of the brain are more active, scientists have been able to map the areas that get more blood.

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

"During the study we observed a really fascinating pattern of neural activity," notes the neuroscientist.

A gag activates not just the areas of the mind responsible for auditory processing and interpreting language, but also neural regions associated with both preparation and starting movement and those linked to vision and memory.

Combine these elements as a whole, and people listening to a pun have a sophisticated set of neural responses that underpin the laughter we hear.

The Infectious Power of Laughter

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

"This was in areas of the mind that you would use to contort your face into a smile or a laugh," she says.

It means we are not just responding to humorous words, they are responding to the laughter that follows them.

Amusement, according to the expert, can be contagious.

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

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

When it comes to festive cracker puns, she says, the positive factor is more probable to be triggered not by the gag itself, but from the reaction to it.

"It's the laughter. The joke is the dreadful holiday cracker pun, and it's just a reason to chuckle together."

The Quest for the Ideal Festive Pun

Is it possible to discover the ultimate gag?

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

In 2001, a psychologist established a scientific project for the world's most humorous joke.

Over tens of thousands of gags later, with ratings provided by hundreds of thousands of participants around the world, he has a clearer idea than most as to what works and what does not.

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

"But they also need to be bad gags, puns that make us groan," he continues.

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

"The reason is that if no-one laughs – it's the joke's shortcoming, not yours.

"The fascinating part about the holiday cracker jokes is that not one person considers them humorous.

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