  The magnetic flux freezing theorem or Alfv\'en's theorem, is a basic
principle of ideal magnetohydrodynamics (MHD), a commonly used approximation to
describe the aspects of astrophysical and laboratory plasmas. The theorem
states that the magnetic flux---the integral of magnetic field penetrating a
surface---is conserved in time as that surface is distorted in in time by fluid
motions. Pedagogues of MHD commonly derive flux freezing without showing how to
take the material derivative of a general flux integral and/or assuming a
vanishing field divergence from the outset. Here I avoid these shortcomings and
derive flux freezing by direct differentiation, explicitly using a Jacobian to
transform between the evolving field-penetrating surface at different times.
The approach is instructive for its generality and helps elucidate the role of
magnetic monopoles in breaking flux freezing. The paucity of appearances of
this derivation in standard MHD texts suggests that its pedagogic value is
underappreciated.
