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

This joke is greeted with groans that resonate through a warehouse in London.

We're at a humor-evaluation meeting with a firm that makes supplies for social events. Its repertoire includes Christmas crackers.

The firm's owner smiles, almost apologetically at the gag. But the pun has been selected and will feature in future crackers.

"You measure the gag by the volume of groans and the loudness of the groans at the table," she explains.

The key to a great Christmas cracker joke is not the same as a stand-up gag per se. It is entirely about the setting - in this case, the communal amusement of the Christmas dinner table with elders, children and potentially friends.

"The goal is for the gag to be something that brings the eight-year-old together with the grandparent," she adds.

The Science Behind Shared Amusement

Coming together to experience shared laughter is not only ancient, scientists say, it is probably to be pre-human.

"Therefore when you are laughing with people at the Christmas dinner you are engaging in what's very likely a really primordial mammal social vocalisation," explains a neuroscience expert.

Shared amusement, she explains, aids in make and maintain social bonds between people.

Scientists have found that a lack of such interactions can seriously harm mental and physical health.

"The people you converse with, and laugh with, it results in enhanced levels of endorphin release," she adds.

These natural chemicals are the brain's "feel-good compounds" and are released both to reduce tension and discomfort and in response to pleasurable experiences, such as laughing with friends over a particularly awful festive cracker joke.

"It's not simply laughing at a silly pun with a Christmas cracker," the expert says. "You are in fact doing a lot of the really important work of building, preserving the connections you have with those you care about."

What Happens In the Brain?

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

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

Using brain scanning technology, a kind of neural imager which indicates which parts of the mind are working harder, researchers have been able to chart the regions that receive more blood.

Testing involves imaging the minds of healthy subjects and then subjecting them to a collection of humorous phrases, accompanied by either a non-emotional sound, or pre-recorded laughter.

"During the study we got a very fascinating pattern of neural activity," says the professor.

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

Put all of this as a whole, and individuals hearing a joke have a complex set of neural responses that support the laughter we hear.

The Contagious Power of Chuckles

Researchers found that when a funny phrase is combined with laughter there is a stronger response in the mind than the same word when accompanied by a non-emotional sound.

"This activation occurred in areas of the brain that you would use to contort your expression into a grin or a chuckle," the professor says.

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

Amusement, says the expert, can be infectious.

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

"People laugh harder when you are familiar with others," she notes, "and you laugh further when you are fond of them or care for them."

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

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

The Quest for the Perfect Festive Pun

Is it possible to find the ultimate gag?

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

Years ago, a professor set up a research project for the world's most humorous joke.

Over tens of thousands of gags later, with scores provided by 350,000 people globally, he has a better understanding than many as to what works and what does not.

The perfect Christmas cracker joke needs to be brief, he says.

"They must also be bad gags, jokes that make us groan," he adds.

The increasingly "awful" the gag, he says the more effective.

"This is because if no-one finds it funny – it's the joke's fault, not your own.

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

"It creates a common experience at the table and I think it's lovely."