There are some chemical elements that expand when heated and contract when cooled. When most metals are heated, they expand. The expansion causes an increase in their volume. At the same time when they cool down, they contract or get smaller. This property of the materials makes them very useful in many applications. One interesting example that proves this fact is the Eiffel tower. In summer, expansion due to heat causes the tower to be 15cms taller.
When a substance is heated, the kinetic energy of the molecules of the substance increases. Its molecules and atoms start to move about more. Now the substance is said to occupy more space than before. When the same substance is cooled down, the kinetic energy decreases. So the atoms and molecules take up less space. Whether the molecules expand or contract, their molecules stay the same size. It should be noted that it is only the space between them, that changes on heating or cooling. Some metals expand more than others due to differences in the forces between the atoms and molecules.
All metals do not expand equally when heated through the same range of temperature. In metals such as iron the forces between the atoms are stronger so it is more difficult for the atoms to move around. In brass the forces are a little weaker so the atoms are free to move about more.
The property of expansion and contraction normally affects the concrete structures. Be it a bridge, building or even a highway, the effect of changes in temperature has an effect on it. Engineers must be very careful while designing and building large metal structures. In order to compensate for the change in the structure due to expansion and contraction, concrete joints are used. If there are no provisions to heat dissipation, severe problems would develop in massive structures. No wonder the Eiffel tower which is made of iron with about 18,000 metal parts grows taller in summer due to the metal expansion.