Phases of Matter
There are three common phases of matter: solid, liquid and gas. They can each be defined by the behavior of their particles (atoms, ions or molecules).
In a solid, particles are locked in place relative to each other by the intermolecular attractions. This is why solid retain their shape and size.
In a liquid, particles are held together by the intermolecular attractions but can move relative to each other. This is what allows liquids to take the shape of their container.
Gas particles behave independently. Generally gas particles are very far apart (relative to their size), and they move randomly. This is why gases take the shape of their container and fill whatever container they are in.