Here's why:

* Solid: Solids have a definite shape and volume. Strings maintain their shape and don't readily change their volume.

* Liquid: Liquids take the shape of their container and have a definite volume. Strings don't flow like liquids.

* Gas: Gases have no definite shape or volume and fill their container. Strings are clearly not gases.

Let me know if you have other questions about the states of matter!