For decades biochemists have been teasing apart the metabolic circuits that power eukaryotic cells. Their descriptions of how enzymes work and play together have illuminated everything from carbohydrate metabolism to DNA replication. Yet technical limitations have forced these scientists to record the behavior not of individual proteins in single cells, but of populations of proteins in millions of cells. The result is an average, idealized description of intracellular behavior, which is the functional equivalent of trying to understand what a group of people is thinking by listening to the roar of a crowd.