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.
In general, when considering the states of matter, solids are more dense than liquids and liquids are more dense than gases. Water is a bit of a contrarian in this regard. When water is in its solid state (ice), the water molecules are packed close together preventing it from changing shape.