  Stellar evolution models calculate convective boundaries using either the
Schwarzschild or Ledoux criterion, but confusion remains regarding which
criterion to use. Here we present a 3D hydrodynamical simulation of a
convection zone and adjacent radiative zone, including both thermal and
compositional buoyancy forces. As expected, regions which are unstable
according to the Ledoux criterion are convective. Initially, the radiative zone
adjacent to the convection zone is Schwarzschild-unstable but Ledoux-stable due
to a composition gradient. Over many convective overturn timescales the
convection zone grows via entrainment. The convection zone saturates at the
size originally predicted by the Schwarzschild criterion, although in this
final state the Schwarzschild and Ledoux criteria agree. Therefore, the
Schwarzschild criterion should be used to determine the size of stellar
convection zones, except possibly during short-lived evolutionary stages in
which entrainment persists.
