Substances expand (increase in size) when they get warmer, and they contract (decrease in size) when they get cooler. This property can be useful. For example:
The expansion and contraction of materials can also cause problems. For example, bridges expand in the summer heat and need special joints to stop them bending out of shape.
When substances expand or contract, their particles stay the same size. It is the space between the particles that changes:
Remember, the particles themselves never expand.