The Kinetic Molecular Theory states that all matter is composed of particles in constant motion. It explains the properties of solids, liquids, and gases by making five key assumptions: (1) gases consist of particles far apart with mostly empty space, (2) collisions between particles are perfectly elastic, (3) gas particles move in random motion with kinetic energy, (4) there are no attractive or repulsive forces between particles, and (5) the average kinetic energy of particles depends on temperature. The theory accounts for the physical properties of gases and is useful for describing ideal gases.