New York

JONAH Berger, an associate professor of marketing at the Wharton School of the University of Pennsylvania, is the author of Contagious: Why Things Catch On. The word "cool" has been cool for a long time. Originally associated with temperature, by the 16th century the term had evolved to describe not just the atmosphere, but also an internal state of calm, almost icy composure. And by the late 1800s, it began to signify style and hipness and some of the other meanings with which it is associated today. Now, cool is used as a synonym for almost anything good. Music can be cool and restaurants can be cool. Every so often, even a minivan seems cool.

But not all words and phrases persist. In the 1940s, dress snappy and someone might say you looked "spiffy". In the 1950s, people might say you looked "swell". These days, teenagers might say you're "on fleek". What was once "awesome" is now "dope". Tell someone today that they look spiffy and people will think you're caught in a time warp.