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

This joke is greeted with groans that echo through a storage facility in London.

This describes a humor-evaluation session with a company that makes products for gatherings. Its repertoire includes Christmas crackers.

The firm's founder grins, nearly apologetically at the gag. But the joke has made the cut and will feature in future crackers.

"The success is gauged by the joke by the number of groans and the loudness of the groans at the table," she explains.

The key to a great Christmas cracker pun is not the identical as a stand-up joke per se. It is all about the setting - in this case, the communal amusement of the Christmas meal with elders, children and possibly neighbours.

"You want the gag to be something that brings the child in harmony with the 80-year-old," she adds.

The Science Behind Shared Laughter

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

"So when you are laughing with others at the Christmas dinner you are dropping into what's almost certainly a truly primordial mammalian social vocalisation," explains a professor.

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

Researchers have found that a lack of such social exchanges can seriously harm mental and physical health.

"The people you talk to, and laugh with, it results in increased amounts of endorphin uptake," she adds.

These natural chemicals are the body's "feel-good compounds" and are released both to alleviate tension and discomfort and in reaction to pleasurable activities, such as chuckling with loved ones over a truly terrible Christmas cracker joke.

"It's not simply laughing at a foolish pun with a Christmas cracker," she says. "You are in fact performing a lot of the really vital task of building, preserving the social bonds you have with those you care about."

Which Happens Inside the Brain?

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

An awful lot happens in response to humour, it transpires.

Using brain scanning technology, a type of brain scanner which shows which areas of the brain are working harder, researchers have been able to chart the regions that get more blood.

The research involves imaging the minds of volunteer participants and then exposing them to a collection of funny words, paired with either a non-emotional sound, or pre-recorded laughter.

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

A joke activates not just the parts of the mind in charge of hearing and interpreting language, but also neural areas involved in both preparation and initiating motion and those involved in sight and memory.

Combine all of this together, and people hearing a joke have a complex series of brain responses that support the laughter we hear.

The Infectious Power of Laughter

Scientists discovered that when a funny word is combined with laughter there is a greater reaction in the mind than the same phrase when accompanied by a neutral sound.

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

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

Laughter, according to the expert, can be contagious.

So what does this mean for the laughter heard around a Christmas gathering?

"People laugh more when you know others," she says, "and laughter increases more when you like them or love them."

When it comes to Christmas cracker puns, she explains, the positive effect is more probable to be caused not by the gag itself, but from the response to it.

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

The Quest for the Ideal Festive Pun

Will we ever discover the perfect gag?

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

Years ago, a psychologist set up a scientific project for the planet's most humorous gag.

Over tens of thousands of gags submitted, with ratings lodged by hundreds of thousands of people globally, he has a clearer idea than most as to what works and what does not.

The ideal festive cracker joke must be brief, he says.

"But they also need to be poor jokes, jokes that cause us to moan," he continues.

The increasingly "terrible" the joke, he states the more effective.

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

"What's interesting about the holiday cracker puns is that none of us considers them humorous.

"That's a common moment around the gathering and I believe it's wonderful."