This site allows you to check the strength of your password using a database of weak and common passwords, and evaluates passwords using several criteria such as the number of characters, combinations of characters and uniqueness...
Teaching situation: enter a password (be careful, not a real one that you use! You never know if a nasty hacker was behind the site and stole your password...) and evaluate its robustness. After this experiment, take some time to find good practices and advices about safe passwords.