What evidence did scientists use to figure out that Earth’s inner core is solid rock?

magnetic field.

Explanation:

scientists started from the known fact that the Moon and Sun cause tides in the liquid outer core, just as they do on the oceans on the surface. He observed that motion of the liquid through the local magnetic field creates electric currents, that dissipate energy as heat according to Ohm's law.

a substance in a physical state in which it does not resist change of shape and will expand indefinitely to fill any container.

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