Here's why:
* Solids: Particles in solids are tightly packed together and vibrate in fixed positions. They have very little freedom to move around.
* Liquids: Particles in liquids are more spread out than solids and can move around each other. They have more freedom of movement than solids.
* Gases: Particles in gases are very spread out and move freely and randomly. They have the most freedom of movement.
So, the less movement, the more tightly packed the particles are, and that's the defining characteristic of a solid.
