My mother has this friend whose daughter got sick from rat pee on her soda can.

Sound familiar? You've might have heard the same story. Except that it was someone's boyfriend's brother — or friend's cousin, or doctor's travel agent — who became ill. Either our food inspection system has gone downhill fast, or the story is an urban legend.

Urban legends are an important part of popular culture, experts say, offering insight into our fears and the state of society. They're also good fun.

"Life is so much more interesting with monsters in it," says Mikel J. Koven, a folklorist at the University of Wales. "It's the same with these legends. They're just good stories."

The making of a legend

Like the variations in the stories themselves, folklorists all have their own definitions of what makes an urban legend. Academics have always disagreed on whether urban legends are, by definition, too fantastic to be true or at least partly based on fact, said Koven, who tends to believe the latter.

Urban legends aren't easily verifiable, by nature. Usually passed on by word of mouth or — more commonly today — in e-mail form, they often invoke the famous "it happened to friend of a friend" (or FOAF) clause that makes finding the original source of the story virtually impossible.

Discovering the truth behind urban legends, however, isn't as important as the lessons they impart, experts say.

Click here to read the Top 10 urban legends.

"The lack of verification in no way diminishes the appeal that urban legends have for us," writes Jan Harold Brunvand in "The Vanishing Hitchhiker: American Urban Legends and Their Meanings" (W.W. Norton & Company, 1981). "We enjoy them merely as stories, and tend to at least half-believe them as possibly accurate reports."

A renowned folklorist, Brunvand is considered the pre-eminent scholar on urban legends and "The Vanishing Hitchhiker," named for a classic legend, the subject's seminal work. The definition of an urban legend, he writes, is "a strong basic story-appeal, a foundation in actual belief, and a meaningful message or 'moral.'"

Most urban legends tend to offer a moral lesson, Koven agreed, that is always interpreted differently depending on the individual. The lessons don't necessarily have to be of the deep, meaning-of-life, variety, he said.

Legends need to make cultural sense

Urban legends are also good indicators of what's going on in current society, said Koven, who is part of the International Society for Contemporary Legend Research (ISCLR) and is editor of its peer-reviewed journal, Contemporary Legend.

"By looking at what's implied in a story, we get an insight into the fears of a group in society," he told LiveScience. Urban legends "need to make cultural sense," he said, noting that some stick around for decades while others fizzle out depending on their relevance to the modern social order.

It's a lack of information coupled with these fears that tends to give rise to new legends, Koven said. "When demand exceeds supply, people will fill in the gaps with their own information…they'll just make it up."

The abundance of conspiracy theories and legends surrounding 9/11, the war in Iraq and Hurricane Katrina seems to point to distrust in the government among some groups, he said.

A lot of fun, too …

But urban legends aren't all serious life lessons and conspiracy theories, experts say, with the scariest, most plausible ones often framed as funny stories.

Those stories can spread like wildfire in today's Internet world, but they've been part of human culture as long as there has been culture, and Brunvand argues that legends should be around as long as there are inexplicable curiosities in life.

"It might seem unlikely that legends — urban legends at that — would continue to be created in an age of widespread literacy, rapid mass communications, and restless travel," he wrote in "The Vanishing Hitchhiker," printed many years before widespread use of the Internet was common. "A moment's reflection, however, reminds of the many weird, fascinating but unverified rumors that often come to our ears — killers and madmen on the loose, shocking or funny personal experiences, unsafe manufactured products and many other unexplained mysteries of daily life."

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