For decades, the default mode network — the set of brain regions most active when we're not focused on a task — was considered metabolic noise, an idling engine. A landmark study using high-density fMRI and simultaneous behavioral probing has demolished that assumption. The DMN is not resting. It is continuously simulating future social scenarios, running probabilistic models of other people's mental states, and updating those models based on recent experience. The brain is never offline.