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 objects like railroad tracks heat up in the sun, they can expand and may even buckle if there is not enough space for this expansion. Conversely, when temperatures drop, materials contract as their particles slow down and come closer together.