  It is well known that, in the absence of external forces, simple
non-relativistic fluids involve entropy production only through heat conduction
and shear viscosity. In this work, it is shown that a number density gradient
contributes to the local entropy production of a simple relativistic fluid
using special relativistic kinetic theory. Also, the presence of an external
field may cause strictly relativistic contributions to the entropy production,
a fact not widely recognized. The implications of these effects are thoroughly
discussed.
