Ben and Joe tackle the third principle in SOLID: the Liskov Substitution Principle. Learn some basic rules you can apply to subclasses and other "is a" relationships to improve flexibility in your applications and reduce subtle bugs from interface...
This is a companion discussion topic for the original entry at https://thoughtbot.com/upcase/videos/liskov-substitution-principle
The LSP link in the description leads to something completely irrelevant.