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

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

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

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

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

The secret to a good Christmas cracker pun is not the same as a stand-up joke in itself. It is entirely about the setting - in this case, the shared amusement of the Christmas dinner table with elders, children and potentially friends.

"The goal is for the gag to be something that unites the child together with the 80-year-old," she states.

The Science Behind Communal Amusement

Gathering to experience shared amusement is not only nothing new, scientists say, it is likely to be older than humanity.

"Therefore when you are chuckling with others around the Christmas table you are engaging in what's very likely a really ancient mammal play vocalisation," explains a neuroscience expert.

Shared laughter, she says, helps forge and strengthen social bonds between people.

Scientists have found that a lack of such interactions can significantly damage mental and physical well-being.

"Those you talk to, and share laughter with, it leads to enhanced levels of 'happy chemical' release," she adds.

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

"You're not just laughing at a silly joke with a Christmas cracker," she states. "You are in fact doing a lot of the really important task of making, maintaining the connections you have with those you care about."

What Happens Inside the Mind?

But what is actually taking place within the mind when we listen to a joke?

An awful lot occurs in reaction to humour, it transpires.

Using functional magnetic resonance imaging (fMRI), a kind of brain scanner which indicates which areas of the brain are more active, scientists have been able to chart the regions that get more blood flow.

Testing entails scanning the minds of healthy participants and then exposing them to a collection of funny words, accompanied by either a neutral sound, or pre-recorded chuckles.

"In the scanner we observed a very fascinating activation pattern of neural activity," says the professor.

A joke activates not just the areas of the mind in charge of hearing and interpreting language, but also neural regions associated with both preparation and starting movement and those involved in sight and memory.

Put all of this together, and individuals listening to a pun have a sophisticated series of brain reactions that support the laughter we experience.

The Contagious Power of Chuckles

Scientists found that when a funny phrase is combined with chuckles there is a stronger response in the mind than the identical word when accompanied 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," the professor explains.

It means we are not just reacting to funny jokes, they are responding to the amusement that follows them.

Laughter, says the professor, can be contagious.

So what does this mean for the laughter heard at a Christmas table?

"You laugh more when you are familiar with others," she says, "and laughter increases further when you are fond of them or care for them."

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

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

The Quest for the Ideal Cracker Joke

Is it possible to discover the perfect gag?

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

In 2001, a psychologist established a scientific project for the planet's most humorous joke.

More than tens of thousands of gags later, with scores lodged by hundreds of thousands of people globally, he has a better understanding than most as to what works and what fails.

The perfect Christmas cracker joke must be short, he says.

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

The more "awful" the joke, he says the more effective.

"The reason is that if no-one finds it funny – it's the gag's shortcoming, not your own.

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

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