This chapter describes the nature of soap films and films produced by other surfactants. It explains the importance of surface energy and introduces minimal-area surfaces. For the surface of the liquid to be in a state of uniform surface tension, the surface tension must be perpendicular to any line drawn in the surface. The chapter shows that the sudden switch between positions is analogous to a change of phase as studied in thermodynamics and as demonstrated in crystal structures. Josiah Willard Gibbs well-known for his contributions to thermodynamics and statistical mechanics, investigated the draining and thinning of soap films. The minimization of the energy of a soap film is another aspect of the theorem and it is the general nature of the theorem originally proposed by Maupertuis that allows soap films to be used for such extensive illustrations of other aspects of physics.