Here's why:

* Solid: Has a fixed shape and volume.

* Liquid: Has a fixed volume but takes the shape of its container.

* Gas: Has no fixed shape or volume, expands to fill its container.

* Plasma: Is a superheated gas where electrons are stripped from the atoms, creating a charged state. It's the most common state of matter in the universe, found in stars and lightning.

While some might think "gas" is the answer, plasma is considered a distinct state of matter with unique properties.