Solids are a state of matter in which the atoms or molecules are tightly packed together, causing the matter to resist changes to its shape or volume.

Liquids are a state of matter in which the atoms or molecules are loose, yet form a distinct surface at the boundary of the substance.  Liquids take the shape of the container in which they are held.

Gases are a state of matter in which the atoms or molecules are free to move in random motion.  Gases do not form surface boundaries or take the shape of their container.

We'll use water as an example throughout this dialog.  Water can exist in one of three states: solid (ice), liquid (water), or gas (water vapor).  The state of water is considered a physical property because it does not affect the identity of the water.  Water molecules are still water molecules whether the water is solid, liquid, or gas.