Chapter 3 was very intriguing to me. It was the first chapter that lead away from design and more into psychology and ourselves. Norman made complete sense when he differentiated the two types of knowledge: Knowledge in your head and knowledge in the world. He went into further detail when he talked about memory. Memory can be both knowledge in the world and in your head.
Knowledge in your head has more do to with short and long term memory. Knowledge in the world also has a lot to do with memory. The difference is that you are required to use natural mappings, like when he showed the different stove set ups. It made me think of my own stove and how i remember which knob goes to what burner through natural mapping. Usually you don't even realize you are doing this until it is brought to your attention.
Norman also discussed the different types of mental models. This first is what the designer makes, called the designers model. That is what the designer intended it to look like. The second is the user's model. That is what us as user's figure out about how we think the design works. The last is the system image. The system image is the way the product looks at the end when we use it. It often includes it's own manual, telling you how to use it.