I have a question about transfer of electric charge in metals. My understanding is that electrons move in and out of an object at an equal rate when current is flowing (i.e., net change in # of electrons is zero). At the same time, it is possible for an object to become positively or negatively charged by gaining or losing electrons. These two facts seem contradictory to me; if there is no net change in # of electrons, how can any charge be acquired at all? What am I misunderstanding? Thanks.