FRONTLINE journeys into the world of the marketers of popular culture to teenagers. They spend their days sifting through reams of market research data. They conduct endless surveys and focus groups. They comb the streets, the schools, and the malls, hot on the trail of the "next big thing" that will snare the attention of their prey - a market segment worth an estimated $300 billion a year. They are the merchants of cool: the creators and sellers of popular culture, who have made teens the hottest consumer demographic in America. But are these marketers merely reflecting a growing coarseness in teen culture, or have they helped create it? Are they simply reflecting teen desires or have they begun to manufacture those desires in a bid to secure this lucrative market? And have they gone too far in their attempts to reach the hearts - and wallets - of America's youth?