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

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

This describes a humor-evaluation session with a firm that makes products for social events. Its repertoire includes festive crackers.

The firm's owner smiles, almost apologetically at the joke. But the joke has been selected 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 secret to a good Christmas cracker pun is not the identical as a good gag per se. It is entirely about the context - in this case, the shared amusement of the holiday dinner table with grandparents, kids and possibly friends.

"You want the joke to be a thing that brings the child in harmony with the grandparent," she adds.

The Science Of Shared Laughter

Coming together to experience communal laughter is not only nothing new, experts say, it is probably to be older than humanity.

"So when you are chuckling with others at the holiday table you are engaging in what's almost certainly a really primordial mammalian social vocalisation," says a professor.

Communal laughter, she says, helps make and maintain social bonds between individuals.

Researchers have found that a absence of these social exchanges can significantly harm mental and physical well-being.

"The people you converse with, and laugh with, it results in increased amounts of 'happy chemical' uptake," the professor continues.

Endorphins are the brain's "feel-good compounds" and are produced both to reduce stress and pain and in response to enjoyable experiences, such as chuckling with friends over a particularly terrible festive cracker gag.

"It's not simply chuckling at a silly joke with a Christmas cracker," the expert says. "You are actually performing a lot of the really important work of building, preserving the social bonds you have with the people you care about."

What Happens Inside the Mind?

But what is actually taking place within the mind when we hear a gag?

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

Using brain scanning technology, a kind of neural imager which shows which areas of the brain are more active, scientists have been able to chart the regions that receive more blood.

Testing entails imaging the minds of healthy participants and then subjecting them to a collection of funny phrases, paired with either a neutral sound, or pre-recorded chuckles.

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

A joke activates not just the areas of the brain in charge of hearing and interpreting speech, but also neural regions involved in both planning and initiating movement and those involved in sight and recall.

Put all of this as a whole, and people hearing a pun have a sophisticated series of brain responses that underpin the laughter we hear.

The Infectious Power of Laughter

Scientists found that when a humorous phrase is paired with laughter there is a stronger response in the mind than the same phrase when accompanied by a non-emotional sound.

"This was in parts of the brain that you would use to move your face into a grin or a chuckle," the professor explains.

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

Amusement, according to the professor, can be contagious.

So what does this imply for the laughter heard at a holiday table?

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

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

"It's the laughter. The gag is the terrible Christmas cracker joke, and it's just a pretext to chuckle together."

The Quest for the Perfect Festive Pun

Is it possible to find the perfect joke?

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

Years ago, a psychologist set up a research search for the planet's funniest gag.

More than 40,000 gags later, with ratings provided by 350,000 participants around the world, he has a clearer idea than many as to what works and what does not.

The perfect festive cracker pun needs to be brief, he says.

"They must also be bad jokes, jokes that cause us to groan," he continues.

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

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

"The fascinating part about the Christmas cracker puns is that none of us find them humorous.

"It creates a shared experience around the gathering and I think it's lovely."