Rocks are solid due to tightly packed particles, while lava is a liquid that flows to take the shape of its container. Cooling processes determine the formation of mineral crystals within rocks and lava.
A rock is a solid because its particles are tightly packed and have a fixed shape and volume. Whereas, lava is a liquid as it flows to take the shape of its container and has a fixed volume. This difference is due to the cooling process: rock solidifies over time allowing mineral crystals to form, while cooling lava solidifies rapidly, resulting in small mineral crystals or an amorphous structure.
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