  We discuss a numerical 3D radiation-MHD simulation of penumbral fine
structure in a small sunspot. This simulation shows the development of short
filamentary structures with horizontal flows, similar to observed Evershed
flows, and an inward propagation of these structures at a speed compatible with
observations. We conclude that the Evershed flow represents the horizontal flow
component of overturning convection in gaps with strongly reduced field
strength. The top of the flow is always directed outward--away from the umbra--
because of the broken symmetry due to the inclined magnetic field. Upflows
occur in the inner parts of the gaps and most of the gas turns over radially
(outwards and sideways), and descends back down again. The ascending, cooling
and overturning flow tends to bend magnetic field lines down, forcing a
weakening of the field that makes it easier for gas located in an adjacent
layer--further in--to initiate a similar sequence of motion, aided by lateral
heating, thus causing the inward propagation of the filament.
