When metal is heated, the atoms begin to vibrate. This increased motion causes the atoms to move farther apart. When the atoms move farther apart, the metal expands. This tendency for metals to expand when heated is called thermal expansion.
The property of thermal expansion can create challenges when metal is used in real-world applications. When metal is used in an environment where the temperature fluctuates, expansion joints must be included; otherwise, the metal would buckle or separate. Metal bridges often use expansion joints for this reason. Railroad tracks are built so they can slide toward one another as they are heated, allowing railroad cars to continue on their way instead of coming to a halt due to deformity of the metal caused by the heat expansion.
The property of thermal expansion also allows for some unique applications of metal. The contraction and expansion of metal strips in a thermostat allows the strips to turn the heat on and off. Heating a metal lid with hot water can cause the metal to expand, increasing the diameter of the lid. This breaks the seal and allows the jar to be opened. If a metal is heated before it is fit to an object, a tighter fit can be obtained upon cooling. This technique is used by welders in a process called shrink fitting.