Here's why:

* Solids have a definite shape and volume. Their particles are tightly packed and arranged in a fixed pattern.

* Liquids have a definite volume but take the shape of their container. Their particles are close together but can move around more freely.

* Gases have no definite shape or volume. Their particles are far apart and move randomly.

Think of it like this:

* Solid: A block of ice

* Liquid: Water in a glass

* Gas: Air filling a room