Electrons in a molecule zip around the atom in times measured in billionths of a billionth of a second, or attoseconds. To learn the basic mechanisms of chemical reactions – how catalysts work, how batteries operate – we need to follow electrons over these very short times, and for that, we need a stopwatch that ticks in attoseconds. In this lecture, I will explain how we use SLAC's X-ray laser, the Linac Coherent Light Source (LCLS), to measure times in attoseconds. Part of the challenge is to verify that the stopwatch is accurate. This work provides tools to watch electrons move on their natural time scales.