Theory Into Practice
The theory behind Liskov's substitution principle is a little obtuse, and it uses formal language. When you see concepts laid down, you'd realize you have done it before. Concrete examples help developer see the more specific picture, relating to what the principle is talking about.
Liskov's Substitution Principle
Aimee: Reb Conf?, Lily's Sweets Dark Chocolate, and How to Write a Good Vimrc
Cory: In the Plex and focus