One item many new data base designers struggle with is the concept of time-variant data. If you think about it, most database structures (entities and their relationships) describe a “snap shot in time”. So, the idea of capturing data state over time is somewhat foreign to the techniques you've used and understood in the first four chapters.
Review the time-variant data discussion in chapter 5. After doing so, think of an example (other than what is used in the book) where you might use time-variant data. Create a thread with your example (be as specific as possible) and comment on at least two other student's examples.