A common problem in engineering and science is to derive simple equations governing complicated phenomena. Often complicated governing equations are known, but they are too difficult to analyze. Therefore it is desired to simplify them in such a way that the phenomena of interest will still be adequately described, while finer details which are not of interest can be disregarded. An example of a simplified equation is Darcy’s law, which describes flow of a viscous fluid through a porous medium. The more complicated equation for the same phenomenon is the Navier-Stokes equation. As an example of a general method for simplifying equations, we shall show how to derive Darcy’s law from the Navier-Stokes equation.