Abstract
Earth's core consists of a molten outer layer surrounding a solid inner core, but models suggest that earlier in Earth's history, the core was completely molten. In his Perspective, Buffett discusses the current knowledge of heat flows at the core-mantle boundary, which drive Earth's magnetic field. Today's heat flows are reasonably well understood, but the thermal history of Earth's core remains unclear. To determine when Earth's core solidified, better knowledge of the partitioning of radiogenic elements and more reliable estimates of the power requirements for Earth's magnetic field are required.