Gases are one of the main states of matter, along with solids and liquids. In a gas, the particles are spread out and move freely in all directions because they have a lot of energy. Unlike solids and liquids, gases don't have a fixed shape or volume—they expand to fill whatever space they're in. This is why air fills a room and why balloons can change size. Since gas particles move so fast and are far apart, gases can be easily compressed, like when air is pumped into a tire. Some common examples of gases are oxygen, carbon dioxide, and the air we breathe.