By definition, solids have a fixed volume. The atoms or molecules composing a solid are closely packed together and are stabilized by interatomic or intermolecular forces, making them resistant to further compression.
Solids are one of the four commonly observed states of matter. The others are gas, liquid and plasma. Matter in a gaseous state has widely separated atoms or molecules with no regular arrangement and can conform to the size and volume of its container. Matter in a liquid state has more densely packed atoms or molecules but lacks a fixed structure. It resists compression but can still conform to a container's shape. Matter in the plasma state exists as positively charged atomic nuclei and free electrons. Like a gas, plasma does not have a fixed shape or volume but differs from an ordinary gas in that it responds strongly to electromagnetic forces.