Neuroscientists are still debating exactly how a clump of cells manages to track time. The latest idea, called the population clock theory, suggests neurons all over the brain, rather than a central chunk of gray matter, tick away constantly. A stimulus—say the start of a walk to work—triggers neurons to start firing, creating a pattern of lit-up cells that indicates how much time has passed. Each circuit acts as a unique clock; the path might get one cluster while a passing car gets another, creating countless timers in our heads. Scientists think various brain areas read these circuits, which is why you can cross the street, listen to music, and text at the same time.