Lacy,
It's really an open ended question. Back-up plans can be used when things go crazy and we need pertinent material to make up for missed activity. Or it may be for when time is left at the end of a class. Or when we want or need to do a change up to keep students engaged. Or it may an alternate way of explaining a topic, an activity, or performing a task. I guess the trust of the question is "do we always have several good Plan'B's, Plan-C's, Plan-D's, etc."
Barry Westling
