Q. What is an urban legend?

FYI：
Urban legend: A story, which may at one time have been true, that has grown from constant retelling into a mythical yarn.(给出简要定义，便于您理解这种文化现象）

A.
Many people have heard the tale, occasionally alluded to in popular culture but more often imparted as a thing that "really happened to a friend of a friend," of the dotty grandmother who tried to dry off her damp poodle by placing it in the microwave oven. The dog exploded, sad to say, and Grandma has never been quite the same since.
That story isn't true, of course; it's an urban legend, circulating by word of mouth since the 1970s. It conveys a familiar moral message: new technologies can be a hazard, as well as a boon, to humankind (not to mention dogkind). Such an incident could have happened, but we have no real evidence that it did. (来源：英语学习门户网站EnglishCN.com)

Urban legends, then, are apocryphal stories, told as true and plausible enough to be believed, about horrific, embarrassing, exasperating or ironic things that have supposedly happened to real people (see links to more examples below). They often deal with particularly vexing aspects of modern life.

In lieu of evidence, the teller of an urban legend is apt to rely upon good storytelling and the naming of allegedly trustworthy sources (e.g., "a friend of a friend who swears it's true") to bolster its credibility. As a type of folklore — defined as the beliefs, stories and traditions of ordinary people ("folks") — one way of differentiating between an urban legend and other kinds of narratives (popular fiction, for example) is by comparing where they come from and how they are disseminated. Legends tend to arise spontaneously and are rarely traceable to a single author or point of origin. They spread primarily from individual to individual through interpersonal communication, and only in atypical cases through mass media or other institutional means. Because they end up being repeated by many different people in many different places, the stories tend to change over time. Hence, no two versions of an urban legend are ever exactly alike; there can be as many variants as there are tellers of the tale.

The phrase "urban legend" entered the popular vocabulary in the early 1980s with the publication of folklorist Jan Harold Brunvand's earliest books on the subject, beginning with "The Vanishing Hitchhiker" (W.W. Norton: 1981). Although it has become all but synonymous in common parlance with "false belief," the term actually denotes a more complex and subtle social phenomenon having to do with the production and transmission of folk narratives — narratives which are indeed usually false, but which can also, on rare occasions, prove to be true.

In brief, an urban legend will typically exhibit most or all of the following characteristics:

It is a narrative.
Its veracity is questionable.
It is alleged to be true.
It is plausible enough to be believed.
It is of spontaneous (or, at any rate, indeterminate) origin.
It varies in the telling.
It is attributed to a putatively trustworthy secondhand source (e.g., "a friend of a friend," "my sister's hairdresser's husband," etc.).
It is passed from individual to individual orally or in written form (e.g., via fax, photocopy or email).
It may take the form of a cautionary tale.