The kinetic theory model says that all matter is made of tiny particles that are always moving. The three states of matter, solid, liquid and gas, differ in how the particles are arranged and how they move.
In a solid the particles are packed closely together in a regular pattern. Strong forces hold them in place, so they cannot move from place to place. They only vibrate about fixed positions. This is why a solid keeps its shape and its volume.
In a liquid the particles are still close together, but they are arranged randomly and can slide past each other. This is why a liquid flows and takes the shape of its container, but it keeps a fixed volume.
In a gas the particles are far apart and the forces between them are very weak. They move quickly in random directions, so a gas spreads out to fill its container. The particles move faster when the temperature is higher.
