Fluids obey the "Continuum Assumption." This states basically that substances consist of like-minded molecules and that each have space between them. But these like-minded molecules occupy a defined space.
Fluids obey the continuum assumption, however the molecules of a fluid act differently in that they continually collide with one another and other objects. Accurate measurements of such substances can be done only at infinitesimally small points; the substance itself however, continually changes density, temperature, pressure and velocity.