Observations and simulations have found convective cold pools to trigger and organize subsequent updrafts by modifying near-surface temperature and moisture as well as by lifting air parcels at the outflow boundaries. We study the causality between cold pools and subsequent deep convection in an idealized large-eddy simulation by tracking colliding outflow boundaries preceding hundreds of deep convection events. When outflow boundaries collide, their common front position remains immobile, whereas the internal cold pool dynamics continues for hours. We analyze how this dynamics “funnels” moisture from a relatively large volume into a narrow convergence zone. We quantify moisture convergence and separate the contribution from surface fluxes, finding that it plays a secondary role. Our results highlight that dynamical effects are crucial in triggering new convection, even in radiative-convective equilibrium. However, it is the moisture convergence resulting from this dynamics that moistens the atmosphere aloft and ultimately permits deep convection.