While a password should be long and complex so that it will be difficult to break, the password should be easy for you to remember. One way to do this is to take the first letter of each word from a song that you know. Which principle applies to this method of creating a password?
Answer: Simplicity of Design
Check out this tool to see how strong your passwords are. Usually, the longer the password is, the harder it is to crack. How strong is yours? Try making up a password you thing would be strong, is that strong? Using a tool like this to check password strength gives you a good idea how effective your passwords really are.
Bonus Round: Have everyone in your group test their password, the loser has to buy every ice cream! (not actually enforceable, but you get the point)
Tap the Simplicity of Design link above to learn more about it!