The police have issued this warning: "If you are driving after dark and see an oncoming car with no headlights on, DO NOT FLASH YOUR LIGHTS AT THEM!" Why? Because the no-headlights car is being driven by a gang member, and as part of an initiation rite, the first person who flashes him will be hunted down and killed. (But at least the gang member will turn his lights on afterward.)
You've almost certainly heard that famous urban legend, and most likely, you heard it from someone who swore that it was real. (It's not. See snopes.com.) This idea is sticky -- it's memorable and may change the way you behave -- but it's also viral. People love to retell it. (Many sticky ideas aren't viral. Your physics teacher may have come up with a mind-blowing demo for Bernoulli's principle, but chances are you didn't chat it up.)
Viral marketing has become a hip, low-cost way to reach a lot of peop