This lecture is a mathematical investigation concerning the generality of the concept of a fiber-reinforced fluid, in which elastic fibers are embedded in an incompressible medium. Although such a material may seem to be very special, since fibers by definition only respond to changes in their lengths, we show that an arbitrary (possibly inhomogeneous and anisotropic) incompressible linearly elastic material can be represented by a particular fiber architecture involving 15 families each comprised of straight parallel fibers. What is remarkable here is that the fiber architecture can be fixed in advance, and the match to the desired material properties can be made merely by adjusting the stiffness constants of the fibers, this adjustment being made separately at each point of the material in the inhomogeneous case. Thus, we have described, in effect, a programmable fiber architecture that, when embedded in an incompressible medium, is universal within the class of incompressible linearly elastic materials.