Here's why:
* Solids have a fixed shape and volume. Their particles are tightly packed together and vibrate in place, but they don't have enough energy to move freely.
* Liquids have a fixed volume but can flow and take the shape of their container. Their particles are closer together than in gases but have more energy and move around more freely.
* Gases have no fixed shape or volume and can expand to fill their container. Their particles are far apart and move very quickly and randomly.
Therefore, solids have the slowest-moving particles due to their tight packing and low energy levels.
