I’m an observational astronomer, and I use open clusters to calibrate the evolution of stellar rotation. Open clusters are collections of stars that all share the same age. Because a cluster includes stars of many masses, we can define its age fairly well. I observe stars in many open clusters to see how rotation (and other properties) change over time. In particular, I focus on calibrating the evolution of rotation itself, and how magnetic fields and binary companions alter a star’s rotational evolution.