Sprints are time-boxed, short, consistent, and have no goal-altering changes. Time-boxed means that they have limited time during which a task can be accomplished. The time to finish the goal also must be within a short period. Consistent means that if one sprint lasts two weeks, then all sprints in the project will be two weeks. No goal-altering changes mean that once the sprint starts, no changes are allowed. That's why it's a must to define goals before you start the sprint.