Refer to figure.
The lungs expand and contract to cause breathing, even when you are not thinking about it, as well as when you are asleep. It is an automatic process, but we can also take over it manually, like holding your breath or singing.

The way that the lungs are made to expand is through the movement of the diaphragm. The diaphragm is a muscle that sits at the bottom of the lungs, and is highest in the middle. The diaphragm contracts to pull the bottom of the lungs down, creating a slight vacuum inside the - now larger - lungs, which pulls air in, known as inhaling.

The opposite happens to exhale air, the diaphragm relaxing, pushing upwards, which forces air out of the lungs.