  The rotor-router model is a deterministic analogue of random walk invented by
Jim Propp. It can be used to define a deterministic aggregation model analogous
to internal diffusion limited aggregation. We prove an isoperimetric inequality
for the exit time of simple random walk from a finite region in Z^d, and use
this to prove that the shape of the rotor-router aggregation model in Z^d,
suitably rescaled, converges to a Euclidean ball in R^d.
