Here's why:

* Solids have a fixed shape and volume because their particles are closely packed and held together by strong forces of attraction. These forces restrict the particles from moving freely, resulting in a rigid structure.

* Liquids have a fixed volume but can change shape because their particles are not as tightly packed as solids. They have weaker intermolecular forces than solids, allowing for more movement and fluidity.

* Gases have no fixed shape or volume because their particles are widely spaced and have very weak intermolecular forces. The particles move rapidly and randomly, expanding to fill the available space.

Let me know if you'd like to explore more about intermolecular forces!