Here's why:

* Solids: Particles in solids are tightly packed together in a fixed, rigid structure. They have very strong attractive forces between them, which keeps them close and vibrating in place.

* Gases: Particles in gases are widely spaced and move freely. They have weak attractive forces between them, allowing them to move around randomly.

Think of it like this: Imagine a packed dance floor (solid) versus a large, empty field (gas). The people on the dance floor are close together, while those in the field have plenty of space to move around.