Thermal expansion is the process by which materials change their size or volume in response to changes in temperature. When a substance is heated, its particles move faster and tend to spread apart, causing the material to expand. This phenomenon occurs in solids, liquids, and gases, although the degree of expansion varies among different materials. For example, when metal is heated, it expands, which is why railroad tracks have gaps between sections to accommodate this change. Conversely, when materials cool down, they contract, returning to their original size. Understanding thermal expansion is important in engineering and construction to prevent structural damage.