Particles exist in three states - solid, liquid, and gas. Solids expand when heated and contract when cooled. Liquids contract when cooled and expand when heated, though water is unique in expanding when cooled until it reaches 4 degrees Celsius. Gases contract when cooled and expand when heated, with gases expanding more than liquids when subjected to the same temperature change. Explosions occur when a gas is made to expand rapidly, creating powerful outward forces.