The key idea is that Earth's magnetic field comes from the motion of conducting fluid inside the planet, a geodynamo process. In the outer core, liquid iron-nickel remains molten and can flow. Heat from the inner core and the release of light elements during inner-core solidification drive convection, so parcels of conducting fluid rise and sink. As these conducting motions occur in a rotating frame, the Coriolis force organizes the flow into spiraling, column-like patterns. This moving conductors through any existing magnetic field generate electric currents, and those currents produce magnetic fields of their own. The interaction between flow and field can sustain a dipolar magnetic field over long timescales. The mantle is solid and only weakly conductive, so it doesn’t support the vigorous, sustained convective flow needed to power a global magnetic field. The inner core is solid and also doesn’t provide the large-scale convective motion required for the geodynamo. The crust is thin and not sufficiently conductive to drive the global dynamo. Therefore, the region responsible for generating Earth's magnetic field through convection is the liquid outer core.