Instantly turning a material from opaque to transparent, or from a conductor to an insulator, is no longer the stuff of science fiction. For several years now, scientists have been using lasers to control the properties of matter at extremely fast rates: during one optical cycle of a light wave. But because these changes occur on the timescale of attoseconds—one-billionth of one-billionth of a second—figuring out how they unfold is extremely difficult.