A significant part of aeronautical science is a branch of dynamics called aerodynamics, which deals with the motion of air and the way that it interacts with objects in motion, such as an aircraft.
When you pour a liquid from one container to another, the liquid takes the shape of the container. The volume of the liquid is the same. The properties that solids and liquids share are that they both have definite volume.
The reason for this is because of the state of air and water. Air is a gas state while water is a liquid. A gas has molecules that move around a lot with a ton of space in-between each molecule. A liquid however has molecules that are much closer together.