the postal analogy to understand the idea of stackconsider an analogy provided by the postal service many peoplewhen they get their mailtoss it onto stack on the hall table or into an "inbasket at work thenwhen they have spare momentthey process the accumulated mail from the top down firstthey open the letter on the top of the stack and take appropriate action--paying the billthrowing it awayor whatever after the first letter has been disposed ofthey examine the next letter downwhich is now the top of the stackand deal with that eventuallythey work their way down to the letter on the bottom of the stack (which is now the topfigure shows stack of mail this letter processed first newly arrived letters placed on top of stack figure stack of letters this "do the top one firstapproach works all right as long as you can easily process all the mail in reasonable time if you can'tthere' the danger that letters on the bottom of the stack won' be examined for monthsand the bills they contain will become overdue