The basic method for solving differential equations in a computer is finite differencing. The nicest feature of the method is that it allows analysis of objects of almost any shape, such as earth topography or geological structure. Ordinarily, finite differencing is a straightforward task. The main pitfall is instability. It often happens that a seemingly reasonable approach to a reasonable physical problem leads to wildly oscillatory, divergent calculations. Luckily, a few easily learned tricks go a long way, and we will be covering them here.