According to the physicist Richard Feynman, a system is in equilibrium when “all the fast things have happened but the slow things have not” (1). This definition really applies to a system at steady state, which can either be in thermodynamic equilibrium or in a nonequilibrium steady state. Most systems in nature are not in equilibrium; they exchange fluxes of matter or energy with their surroundings or undergo chemical reactions. When the fast “things” have happened but the slow ones have not, such systems are in a nonequilibrium steady state. The properties of nonequilibrium steady states are currently under intense theoretical investigation, and their similarities and differences with thermodynamic equilibrium states are starting to emerge (2). On page 604 of this issue, Battle et al. (3) propose a new way of probing the nonequilibrium nature of an apparent steady state and demonstrate how such nonequilibrium dynamics can be identified.