  We discuss the Buchert equations, which describe the average expansion of an
inhomogeneous dust universe. In the limit of small perturbations, they reduce
to the Friedmann-Robertson-Walker equations. However, when the universe is very
inhomogeneous, the behaviour can be qualitatively different from the FRW case.
In particular, the average expansion rate can accelerate even though the local
expansion rate decelerates everywhere. We clarify the physical meaning of this
paradoxical feature with a simple toy model, and demonstrate how acceleration
is intimately connected with gravitational collapse. This provides a link to
structure formation, which in turn has a preferred time around the era when
acceleration has been observed to start.
