It hasn't made it into Webster's Dictionary yet, but you already know this word. In fact, it's already in your head. It's that jingle, that logo, that look, that idea. It's called a meme, and there's a whole branch of social science devoted to it. Richard Dawkins, the man who coined the word in his book, "The Selfish Gene," wrote:

"Just as genes propagate themselves in the gene pool by leading from body to body via sperm or eggs, so memes propagate themselves in the meme pool by leaping from brain to brain via a process which, in the broad sense, can be called imitation. . . . Memes should be regarded as living structures, not just metaphorically but technically. When you plant a fertile meme in my mind, you literally parasitize my brain, turning it into a vehicle for the meme's propagation in just the way that a virus may parasitize the genetic mechanism of a host cell."

Viruses are often unwittingly (or maliciously) replicated via e-mail. So are jokes. Friends and colleagues love to forward them. The more timely, the better. (People love David Lettermen's Top 10 because it is of the moment, something they can repeat at the espresso machine the next morning). It's said that pornography makes up a lot of Net traffic, but I bet humor is a close second. We hesitate to forward political rants or poignant thoughts for the day, but give us a knee-slapper, and we'll happily fire it off to a dozen closest friends.