Final answer:
Thermal expansion occurs due to changes in temperature, where the increased kinetic energy of atoms or molecules in a solid pushes neighboring particles apart, resulting in an increase in size. This expansion also occurs in gases and liquids but to a lesser extent.
Step-by-step explanation:
Thermal expansion occurs due to changes in temperature. When the temperature of a substance increases, the kinetic energy of its atoms or molecules also increases. In a solid, the atoms or molecules are closely packed together, but their increased kinetic energy pushes neighboring atoms or molecules apart, resulting in an increase in size. This expansion is not limited to solids, as gases and liquids also undergo thermal expansion, although to a lesser extent.