Is it possible to shoot a goal if the goal post is constantly moving? I think not. So, how does one achieve a moving target? By dreaming. The same principle applies in agile development. If the sprint is not sand boxed, it just becomes a moving target, thus making it imperative to do so. A moving target not only results in loss of valuable momentum but it demoralizes the team as it makes them a failure. Whereas, well-defined sprints at least lead to more predictable outcomes, sustained momentum and sense of achievement, which becomes a self-fulfilling prophecy to achieve success.