The Ball & Ring apparatus is a classic physics experiment used to demonstrate thermal expansion in solids. It consists of a metal ball and a ring, precisely sized so the ball can pass through the ring at room temperature. When the ball is heated, it expands due to thermal expansion and no longer fits through the ring. Once cooled, it contracts and passes through again. This experiment visually explains the effects of heat on materials and is widely used in educational settings to teach concepts of thermal physics, material properties, and real-world applications like bridge expansion joints and metal structures.