  We propose a simple quantum field theoretical toy model for black hole
evaporation and study the back-reaction of Hawking radiation onto the classical
background. It turns out that the horizon is also ``pushed back'' in this
situation (i.e., the interior region shrinks) but this back-reaction is not
caused by energy conservation but by momentum balance. The effective heat
capacity and the induced entropy variation can have both signs -- depending on
the parameters of the model. PACS: 04.62.+v, 04.70.Dy.
