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

This one-liner is greeted with moans that echo through a warehouse in London.

We're at a joke-testing meeting with a firm that makes products for social events. Its catalogue includes Christmas crackers.

The firm's founder smiles, nearly apologetically at the joke. But the joke has been selected and will feature in future crackers.

"You measure the gag by the volume of moans and the intensity of the groans around the table," the founder says.

The secret to a great holiday cracker pun is not the same as a good gag in itself. It is entirely about the context - in this case, the shared laughter of the holiday meal with grandparents, kids and potentially neighbours.

"The goal is for the gag to be a thing that unites the child together with the grandparent," she adds.

The Neuroscience Behind Communal Laughter

Coming together to enjoy shared amusement is not only nothing new, scientists say, it is likely to be older than humanity.

"Therefore when you are laughing with others at the Christmas table you are dropping into what's very likely a really primordial mammal social vocalisation," explains a professor.

Communal amusement, she says, helps forge and strengthen social connections between individuals.

Researchers have discovered that a absence of such interactions can significantly harm mental and physical well-being.

"The people you converse with, and laugh with, it results in enhanced amounts of endorphin release," the professor continues.

Endorphins are the body's "feel-good compounds" and are released both to alleviate stress and pain and in reaction to enjoyable activities, such as laughing with friends over a particularly terrible festive cracker gag.

"It's not simply laughing at a foolish pun with a Christmas cracker," she says. "You are actually doing a lot of the truly vital task of building, preserving the connections you have with those you care about."

What Happens Inside the Mind?

But what is actually taking place inside the brain when we listen to a joke?

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

Employing functional magnetic resonance imaging (fMRI), a kind of brain scanner which indicates which areas of the mind are working harder, researchers have been able to chart the regions that get more blood flow.

Testing entails scanning the brains of healthy subjects and then exposing them to a collection of humorous words, accompanied by either a non-emotional sound, or pre-recorded chuckles.

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

A joke activates not just the areas of the brain responsible for hearing and interpreting speech, but also brain regions associated with both preparation and starting movement and those linked to sight and recall.

Put these elements as a whole, and people listening to a joke have a sophisticated series of brain responses that support the laughter we experience.

The Contagious Nature of Chuckles

Researchers found that when a humorous word is paired with laughter there is a greater reaction in the mind than the same word when followed by a neutral sound.

"This was in areas of the mind that you would use to move your expression into a grin or a chuckle," the professor explains.

It indicates we are not just responding to humorous words, they are reacting to the amusement that follows them.

Laughter, according to the expert, can be contagious.

So what does this mean for the laughter found around a Christmas table?

"You laugh harder when you are familiar with people," she notes, "and laughter increases further when you like them or love them."

When it comes to Christmas cracker jokes, she explains, the positive factor is more likely to be triggered not by the joke itself, but from the response to it.

"The laughter is key. The joke is the terrible holiday cracker joke, and it's just a pretext to chuckle as a group."

The Search for the Ideal Festive Pun

Is it possible to discover the perfect gag?

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

Years ago, a professor established a scientific search for the world's most humorous joke.

Over tens of thousands of gags later, with scores lodged by hundreds of thousands of participants globally, he has a clearer idea than most as to what works and what fails.

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

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

The more "terrible" the gag, he states the more effective.

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

"The fascinating part about the holiday cracker puns is that not one person find them funny.

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