Here's why:

* Solids have a fixed shape and volume. Sunglasses maintain their shape and size, even when you take them off and put them down.

* Liquids take the shape of their container and have a fixed volume. Sunglasses don't do this.

* Gases have no fixed shape or volume, expanding to fill their container. Sunglasses don't behave this way.

Sunglasses are made of solid materials like plastic or metal.