Why do moving liquid metals generate a magnetic field? The earth's magnetic field is generated due to the movement of liquid metals, but why not solid? Why is it that moving molten metals create a magnetic field, but when nothing of the sort happens when I move a metal rod?
 A: You may really be surprised, but you do create both electric and magnetic fields when you move a metal rod.
But,  here is a simple and intuitive example:
An electric generator. It is made of solid metals, but it has bearings and the rotor can rotate. Rotate it fast enough and you get both magnetic fields (inside) and electric fields (at the wires).
The Earth's interior works in the similar manner.
First, imagine that the core is solid (and conductive), but there is a thin layer of lubrication that allows it to rotate in respect to the rest of the Earth's mass. And, imagine some monstrous mechanism that rotates it. Bingo - we have a generator.
We do have the monstrous mechanism required, together with the lubrication, 2 in 1.
We are lucky to have both a liquid-ish outer core and a heat source inside. A convective heat exchange emerges.
The convection constantly mixes not only the masses of different temperature, they have different momentums as well. When a mass plumes upwards, coriolis forces make it lag in regard to the Earth mantle. When the same mass accelerates to the mantle angular velocity (and cools), it plunges down and the same coriolis forces make it advance in regard to the core.
This is why the inner core has somewhat higher angular velocity.
This "motor" is powered by the temperature difference between the internal core and the lower mantle.
(Yes, it is AC. The fields reverse once in a half million years approximately).
