A slightly modified version of this applet is used to illustrate the context of my recent paper with Chao Tang and Yi-Cheng Zhang.
Sand is lost from the pile at the boundaries.
At this point it is time to add another grain of sand. Starting with an arbitrary configuration and repeating the above procedure brings the system to a stationary state, where for every grain of sand added to the system on average precisely one grain of sand is lost at the boundary. It is clear that the system in this state must have large avalanches. Notice that this critical state is a unique attractor of the dynamical rules, conserving sand everywhere except for the boundaries. Hence the name Self-Organized Criticality.