Thermal expansion is the tendency of most materials to increase in size when their temperature rises. As a substance is heated, its particles gain kinetic energy and start to vibrate more vigorously. This increased motion pushes the particles slightly farther apart, causing the material to expand in length, area, or volume depending on its shape.
Solids, liquids, and gases all experience thermal expansion, but the effect is most noticeable in gases due to their loosely packed particles. In everyday life, this principle explains why metal bridges have expansion joints, why overhead power lines sag more in summer, and why liquid thermometers rise with temperature.
The degree of expansion depends on the material’s thermal expansion coefficient—metals like aluminum expand more than materials like glass. Though usually small, thermal expansion must be considered in engineering, construction, and manufacturing to avoid structural damage or malfunction.