When creating educational materials, using the work of others has copyright (legal) issues that are important to understand.
Public Domain - works are free to use
Fair Use is a doctrine in the U.S. Copyright law that allows limited use of copyrighted material without requiring permission.
Fair use is determined by examining four factors:
Giving credit does not absolve you of copyright violations. While proper citation (credit) should be provided for quoted or paraphrased passages or images, it does not automatically make the use fair. The four factor analysis (above) still must be used.
Password-protected sites or course management systems - Placing materials with potential copyright violations on a password protected web site or password protected course management system (e.g. Blackboard) does not make a use fair or avoid copyright issues. Just because it could be hard for someone to discover that copyright infringement is taking place, does not make infringement okay. You still need to use the Four Factor Fair Use test to determine if permission is necessary. Remember the likelihood of getting caught is NOT the same as the likelihood of infringement!