A fully resolved shallow cumulus cloud simulation (albeit at a lower Reynolds number than in the atmosphere) is performed in a quiescent environment to investigate instantaneous entrainment and detrainment. Despite constant boundary conditions and environment, the cloud displays cyclic puff-like behaviour, indicating that puffs are an intrinsic feature of the system not linked to thermals in the sub-cloud region. The cloud dynamics are examined via both Reynolds-averaged statistics and conditionally averaged statistics. The Reynolds-averaged statistics reveal that the majority of entrainment occurs over the upper part of cloud and that the cloud creates an intrusion at about its half height (about 1,000 m) through which it detrains. Using a novel technique that enables direct evaluation of instantaneous entrainment and detrainment fluxes, we examine instantaneous entrainment and detrainment at two interfaces: (1) the cloud boundary, which separates the cloud from the environment; and (2) the updraught boundary, which separates the rising flow (updraught) from the descending flow (subsiding shell). The data show that the entrainment and detrainment rates are a factor of 2 larger than those estimated from the bulk assumption, consistent with other studies. Furthermore, entrainment is strongly correlated to the rising puffs, as evidenced by conditioning the statistics on the instantaneous buoyancy.