"YOU HAVE CHANGED MY LIFE" is a common refrain in the emails Walter Lewin receives daily from fans who have been enthralled by his world-famous video lectures about the wonders of physics. "I walk with a new spring in my step and I look at life through physics-colored eyes," wrote one such fan. When Lewin's lectures were made available online, he became an instant YouTube celebrity, and The New York Times declared, "Walter Lewin delivers his lectures with the panache of Julia Child bringing French cooking to amateurs and the zany theatricality of YouTube's greatest hits."