Here's why:

* Gases have very weak intermolecular forces, meaning the particles are far apart and move freely. This allows gases to expand to fill any container they are in, making their volume easily changeable.

* Liquids have stronger intermolecular forces than gases, but their particles can still move around somewhat. While liquids can change shape, they are not as easily compressed as gases.

* Solids have very strong intermolecular forces, holding their particles in fixed positions. Solids have a definite shape and volume, and they resist changes to either.