  It is well known that suspensions of particles in a viscous fluid can affect
the rheology significantly, producing a pronounced non-Newtonian response even
in dilute suspension. However, it is unclear a priori which particle shapes
lead to this behaviour. We present two simple symmetry conditions on the shape
which are sufficient for a dilute suspension to be Newtonian for all strain
sizes and one sufficient for Newtonian behavior for small strains. We also
construct a class of shapes out of thin, rigid rods not found by the symmetry
argument which share this property for small strains.
