 When we move to physics, it becomes even more difficult—given the highly mathematized nature of the subject—to distinguish between the mathematical and the physical components of an explanation.  Consider the following example. Mark the faces of a tennis racket with R (for rough) and S (for smooth). Hold the tennis racket horizontally by its handle with face S facing up. Let y be the intermediate principal axis. This is the vertical axis perpendicular to the handle and passing through the center of mass of the racquet. Toss the racket attempting to make it rotate about the y axis. Catch the racket by its handle after one full rotation. The surprising observation is that the R face will almost always be up (one would expect S to be up). In other words, the racket makes a half twist about its handle. An explanation of this phenomenon was given in Ashbaugh, Chicone, &amp; Cushman 1991.  They say: “In this paper we explain the twist by analyzing the [differential] equations of motion of the tennis racket in space…Our treatment of the twist is divided into two parts. In the first part we prove two theorems which show that the handle moves nearly in a plane and rotates nearly uniformly…In the second part, we discuss how the twist and rotation of the handle are related” (Ashbaugh &amp; others 1991, 68). There is no question that we are explaining a physical regularity but mathematics enters here both in the modeling of the phenomenon and in the explanatory account by means of the classical dynamics of a rotating tennis racket.