Drawing a mark can be an efficient command input technique when using a pen-based computer. However, marks are not intrinsically self-explanatory as are other interactive techniques such as buttons and menus. We present design principles for interaction mechanisms which make marks self explanatory for novices but still allow experts to use efficient command marks. The key notion is that use of the explanation mechanism physically trains a novice to use the efficient command marks. Two novel interaction mechanisms we have developed using these principles are presented.