Several months ago, I had the very rare chance of seeing
an urban legend in the formation. While going to a dance club, I
saw a bunch of people laughing hysterically, some even pounding
the bar with their fists until their drinks almost spilled.

"Well, what's so funny? Is it my hair extensions?" I grinned.

"No, you got to hear what happened to this friend of a friend of mine!" giggled one. Once I heard that line, I realized exactly what we had on our hands: an urban legend.

So according to this story, her friend works with children who cannot speak, the autistic, to be precise. Now, autistic children need extra supervision, so of course the story goes that this friend is the only person who supervises about 12 or 13 of these children on a field trip to the local aquarium. Feeling bored, she lets them loose for two hours. After a while, she
decides to get a head count...only to find that one of the children is missing! A panicked search around the aquarium finds the child in perfectly normal condition, complete with his backpack and everything intact. The only problem is that the child is completely and totally sopping wet.

"Where were you?" asks the friend.

No answer; the child is unable to speak. The storyteller's friend shrugs, decides that the child is alright, and that no one would notice if he was wet. Without further ado, the friend of a friend returns the child home. An hour later, the school receives a phone call from the mother.

"Hello?" asks the principal.

"Yes, I have two questions about my son's trip to the aquarium today," replies the mother.

"Go on," says the principal.

"First, why is my son wet? And second, WHY IS THERE A LIVE PENGUIN IN MY CHILD'S BACKPACK?"

This was the first time I had ever heard this particular urban legend, and I regularly research them. Either way, after about 30 minutes of laughter, it dawned on me that this would likely end up being an urban legend at one point or another. Well, either way, I liked the story and told it to several friends. 5 weeks later, apparently word had spread - I told the story to a stranger, who apparently heard it from someone else!
This brings me to my main point. What usually becomes an urban legend? What makes people whisper? Through all my readings on the subject, as well as bearing witness to the actual making of an urban legend in my area, I've
noticed several things as follows:
1) Urban legends are usually very infectious stories. They are the juicy gossip tidbits, the shocking news that no one wants you to know, or extremely hilarious. A good clue about whether or not it will turn into an urban legend is how badly you would want to retell it to a friend once you hear it.
2) They have a tendency to make people wonder about what they really know about modern society. Think about all the urban legends about the dangers of modern medicine. People want to double check to see what's going on with their medications. Or the food rumors which have cause many a friend of mine to stop going to fast food restaurants.
3) They seem outlandish at first glance. For instance, in
the story above, it is highly unlikely that a single teacher would supervise a large group of autistic children and then let them roam around an aquarium almost completely unsupervised. It is extremely risky and reckless, and had this happened to most moms, a lawsuit would have been in order. At the worst, it would have made the news.
4) The stories most often do not make the news, but somehow still manage to survive, some of them for decades. The best example of this would be the old "alligators in the sewers" tale.
If this sounds a lot like a story that someone may have told you, or about an absolute scientific "fact" that gets passed around from friend to friend, take it with a grain of salt, you have an urban legend on your hands! Interestingly enough, the entire story narrated to you, although it was told to me with a specific location, was mentioned on snopes.com as an indeterminable urban legend which had roots around 2005. Once again, this proves the point that if it sounds unusual, chances are that it might just be an urban legend.