All substances expand when heated, that is, the volume of a substance will increase when it is heated. This happens because of the motion of molecules making up the material. When the material is heated, by supplying heat to it, the kinetic energy of the constituent molecules increase. This causes more rapid vibrations of these molecules, which increases the spacing between them. This causes an expansion of the heated material. An example is what happens when we heat a metal: it expands. A classical example is that of a smaller metal ring and a basketball. The ball will not pass through the metal ring, under normal circumstances. When the metal ring is heated, the ball passes through it, as the metal expands. Interestingly, up to the melting point, the metal will contract once the temperature returns to pre-heated conditions.
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