Substance X is placed in a cup. Its shape does not change. It is then placed on a plate; its shape still does not change.

A solid keeps its own shape when moved from one container or surface to another. Since Substance X does not change shape in either the cup or on the plate, it is a solid. Liquids and gases take the shape of their container or spread out to fill available space.

AI-written and checked against the verified answer.