The physical state of a type of matter depends mostly upon how its ATOMS AND MOLECULES are arranged and how they move.
In a solid, the atoms and molecules are packed very tightly together and move very little.
In a liquid, the atoms and molecules are a little looser than they are in solids, with atoms and molecules able to flow freely around each other but still kept somewhat together.
In a gas, the atoms and molecules are barely kept together and are able to move around freely. This is why gases are able to expand throughout a container.
