Another reason that managing the environment is challenging is because a teacher can never predict everything that will happen in a class. A well-planned lesson may fall flat on its face, or take less time than you expect, and you find yourself improvising to fill class time. On the other hand an unplanned moment may become a wonderful, sustained exchange among students; so you have to drop previous plans and “go with the flow” of their discussion. Interruptions happen continually: a fire drill, a quick drop-in visit from another teacher or from the principal, a call on the intercom from the office. An activity may turn out well, but also end up rather differently than you intended; you therefore have to decide how, if at all, to adjust the next day to allow for this surprise.