When substances are subjected to heat, they expand and this expansion can be an increase in length, size or area, or volume.
Expansion In Solids:
Whenever a solid substance is heated, it expands and when heat is removed, it contracts e.g.
Bar & gauge experiment:
At ordinary temperature, the bar fits into the gauge. When the bar is heated with a bunsen burner, for some time, the bar cannot fit into the gauge anymore, as a result of expansion which is an increase in length, when the bar is allowed to cool down; it fits into the gauge again, showing that contraction has taken place.