What characteristic makes gases unique compared to other phases of matter?

Gases are unique compared to other phases of matter primarily because they are compressible. This characteristic arises from the significant amount of space between the individual gas molecules, which allows them to be pushed closer together when pressure is applied. In contrast, solids and liquids have molecules that are more closely packed and do not easily change volume under pressure.

When a gas is compressed, the molecules can occupy a smaller volume, which is why gases can be easily compressed into containers or even liquids. This property is not a feature of solids, which maintain a fixed shape and volume, nor of liquids, which have a fixed volume but take the shape of their container. Therefore, being compressible defines gases as distinct from solids and liquids and is a critical aspect of their behavior in different conditions.