Particle Theory states that all matter consists of many, very small particles which are constantly moving or in a continual state of motion. The amount the particles move depends on which state the substance is in.
SolidsEditThe particles are closely packed together and have strong bonds. This is why solids keep their shape, and cannot be compressed. The particles don't move apart from each other but they can vibrate - this kind of molecular vibration is heat energy.
LiquidsEditIn liquids the particles are fairly close together and still have strong attractions between each other. The particles in a liquid can move over and around each other. This means that they fill the bottom of any container they are in. Liquids cannot be compressed.
In gases the particles are free to move anywhere and have very little attraction between each other. This means that they will fill any container they are in. Gases can be compressed.