Here's why:
* Liquids have a definite volume but not a definite shape. This means they can take the shape of their container while maintaining their volume. This ability to move and change shape is what allows liquids to flow.
* Solids have a definite shape and volume, making them unable to flow.
* Gases have neither a definite shape nor volume, and they flow easily. However, their flow is often described as diffusion rather than "flowing" in the same way as liquids.
