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

This one-liner is met by moans that echo through a storage facility in London.

We're at a humor-evaluation meeting with a company that produces supplies for social events. Its catalogue features festive crackers.

The company's owner smiles, nearly sheepishly at the gag. But the pun has been selected and will feature in future crackers.

"You measure the gag by the number of groans and the intensity of the groans around the table," she says.

The secret to a good Christmas cracker joke is not the same as a good joke per se. It is all about the setting - in this instance, the shared laughter of the Christmas dinner table with grandparents, children and potentially neighbours.

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

The Science Behind Communal Laughter

Gathering to experience communal amusement is not only ancient, scientists argue, it is likely to be pre-human.

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

Communal laughter, she says, aids in forge and strengthen social bonds between individuals.

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

"Those you converse with, and share laughter with, it leads to increased levels of 'happy chemical' release," the professor adds.

These natural chemicals are the brain's "happy chemicals" and are released both to reduce stress and pain and in response to enjoyable experiences, such as chuckling with friends over a particularly terrible festive cracker gag.

"You're not just chuckling at a foolish joke with a holiday cracker," the expert states. "You are in fact doing a lot of the truly important work of building, preserving the connections you have with those you love."

Which Occurs In the Brain?

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

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

Employing functional magnetic resonance imaging (fMRI), a type of brain scanner which indicates which parts of the brain are more active, researchers have been able to chart the areas that receive more blood flow.

Testing involves scanning the brains of volunteer participants and then subjecting them to a collection of humorous words, accompanied by either a neutral sound, or recorded chuckles.

"During the study we got a really interesting activation pattern of neural activity," notes the professor.

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

Combine all of this as a whole, and individuals listening to a pun have a complex set of neural reactions that support the laughter we hear.

The Infectious Power of Laughter

Scientists discovered that when a funny word is combined with chuckles there is a greater response in the mind than the identical word when accompanied by a neutral sound.

"This was in parts of the mind that you would employ to contort your face into a grin or a laugh," the professor explains.

It indicates people are not just reacting to funny jokes, they are reacting to the amusement that accompanies them.

Amusement, according to the expert, can be contagious.

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

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

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

"It's the laughter. The gag is the dreadful holiday cracker pun, and it's just a reason to laugh as a group."

The Search for the Ideal Cracker Joke

Will we ever find the perfect joke?

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

Years ago, a professor established a research search for the planet's most humorous gag.

More than 40,000 jokes submitted, with scores 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 pun must be short, he explains.

"But they also need to be poor jokes, puns that make us moan," he adds.

The increasingly "terrible" the gag, he says the better.

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

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

"That's a shared moment around the table and I believe it's wonderful."