We have looked at two different ways to model physical systems: as point particles and as extended objects. But neither of these models seems appropriate to describe the behavior of a fluid. Fluids are made up of particles but those particles interact within the fluid in ways that our previous discussion of point particles cannot account for. Similarly, fluids extend over space, taking up some volume, but also do not obey the constraints built into our rigid rotor model. Unlike an object like a disk that is rotating where the position of the particles that make up the disk are fixed relative to each other, the particles in a fluid are able to flow past one another, switching positions in a way that our analysis of extending objects cannot explain. Therefore, we need a new way to model fluids if we want to describe their mechanical properties. As we will see, we can use an ideal fluid to represent the behavior of the particles within a fluid in both static and dynamics situations.