The properties of reformation in perpendicular collisionless shocks are investigated by means of a one-dimensional particle-in-cell simulation. Reformation is associated with ion reflection at the shock ramp and subsequent ion gyromotion in the upstream region. However, if ions are reflected continuously at the ramp, why does the shock reform intermittently at sufficiently high Mach number? The simulations show that the shock potential changes dramatically through the re-formation cycle, so that it is this potential variation which leads to the intermittent response of the shock.