These rules are based on how people view the computer display and interact with it. A lot of useful design ideas can also come from using what people already know. The design assignment is also intended to teach you iteration. The design process is not perfect. If you put the design down, sleep on it and try again the next day, you will find yourself full of new ideas for how to make the design better.
Constraints also make the design better.
These constraints come from users. You will also find yourself changing the design with even better ideas that remove these holes. We are using a very common element that people interact with everyday and asking you to build a paper prototype of the interaction they might have with this object if it were computerized. Many faucets in airports are computerized. People walk up to them, put their hands under then and they turn on.