Here's why:

* Particles are tightly packed: The particles in a solid are very close together and held in a fixed, rigid structure by strong intermolecular forces.

* Limited movement: This means the particles can't move freely from one place to another like in liquids or gases.

* Vibrations: However, the particles *can* vibrate around their fixed positions. These vibrations increase in intensity as the temperature of the solid increases.

Think of it like this: Imagine a bunch of marbles packed tightly in a box. The marbles can't move around freely, but they can vibrate against each other if you shake the box.