• Learn to think about thinking. At the basis of every program lies a human being's mental model. Creating a program which accurately reflects a shared mental model is crucial to building habitable and maintainable systems.
• East-oriented code. Learn how to better understand encapsulation and create code which enforces that the right objects have the right responsibilities.
• Data, Context, and Interaction (DCI). Discover ways to flatten your object structure and create highly cohesive groups of objects that keep related things together, right where you need them.

Additional materials:

• Forwarding, Inheritance, Delegation cheatsheet. Breeze through the quick refrence for forwarding with Inheritance, Consultation, and Delegation.
• Visualization of message handling. Get this cheatsheet giving you a fast understanding with visualizations of how messages are handled by method_missing, SimpleDelegator, DelegateClass, and Forwardable helping you know how and when to use them.