  Locomotion and generation of flow at low Reynolds number are subject to
severe limitations due to the irrelevance of inertia: the "scallop theorem"
requires that the system have at least two degrees of freedom, which move in
non-reciprocal fashion, i.e. breaking time-reversal symmetry. We show here that
a minimal model consisting of just two spheres driven by harmonic potentials is
capable of generating flow. In this pump system the two degrees of freedom are
the mean and relative positions of the two spheres. We have performed and
compared analytical predictions, numerical simulation and experiments, showing
that a time-reversible drive is sufficient to induce flow.
