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

This quip is greeted with groans that resonate through a storage facility in London.

We're at a joke-testing meeting with a company that produces products for gatherings. Its repertoire features Christmas crackers.

The company's founder smiles, almost sheepishly at the joke. But the pun has been selected and will feature in upcoming crackers.

"The success is gauged by the gag by the number of groans and the loudness of the groans around the table," she says.

The secret to a good Christmas cracker pun is not the identical as a good joke per se. It is entirely about the context - in this case, the shared laughter of the Christmas meal with grandparents, children and potentially friends.

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

The Neuroscience Of Shared Laughter

Coming together to enjoy communal laughter is not only ancient, scientists argue, it is probably to be pre-human.

"Therefore when you are laughing with people at the holiday dinner you are dropping into what's almost certainly a really ancient mammalian play sound," says a professor.

Communal amusement, she says, helps forge and strengthen social bonds between people.

Scientists have discovered that a lack of these social exchanges can significantly damage both psychological and bodily well-being.

"Those you talk to, and share laughter with, it leads to increased amounts of endorphin release," she continues.

These natural chemicals are the body's "happy chemicals" and are produced both to reduce stress and pain and in reaction to enjoyable activities, such as laughing with friends over a particularly awful Christmas cracker gag.

"You're not just chuckling at a silly pun with a Christmas cracker," the expert states. "You are actually performing a lot of the really vital task of making, maintaining the social bonds you have with the people you care about."

Which Happens In the Mind?

But what is actually happening within the brain when we hear a gag?

A tremendous amount happens in reaction to comedy, it transpires.

Using brain scanning technology, a type of brain scanner which indicates which areas of the mind are more active, scientists have been able to chart the areas that receive more blood.

The research involves scanning the brains of volunteer subjects and then subjecting them to a collection of funny words, paired with either a non-emotional sound, or pre-recorded chuckles.

"In the scanner we got a very interesting activation pattern of activation," notes the neuroscientist.

A gag stimulates not just the areas of the mind responsible for hearing and understanding language, but also neural areas associated with both planning and starting movement and those involved in sight and recall.

Combine these elements as a whole, and people hearing a pun have a sophisticated set of neural responses that underpin the amusement we experience.

The Contagious Power of Laughter

Scientists found that when a funny phrase is paired with chuckles there is a greater response in the brain than the identical phrase when followed by a non-emotional sound.

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

It means people are not just responding to humorous jokes, they are responding to the amusement that follows them.

Amusement, says the expert, can be infectious.

So what does this mean for the chuckles found at a Christmas gathering?

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

When it comes to festive cracker jokes, she says, the positive effect is more likely to be triggered not by the joke in itself, but from the reaction to it.

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

The Search for the Ideal Festive Pun

Will we ever find the perfect joke?

Likely not, but that has not stopped experts from attempting to.

In 2001, a professor established a scientific search for the planet's funniest gag.

Over tens of thousands of gags submitted, with scores lodged by 350,000 people globally, he has a clearer idea than many as to what works and what fails.

The perfect festive cracker pun must be short, he says.

"But they also be bad jokes, puns that cause us to groan," he continues.

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

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

"What's interesting about the holiday cracker puns is that not one person find them funny.

"That's a common experience at the gathering and I believe it's lovely."