  : The reason Step 2 needs explanation is because it needs explanation. I gave the explanation. You will notice that the explanation refers to our ignorance of the host's door opening procedure. Till then we had only made use of our ignorance of the car's location. Till then we had only obtained a simple solution (A: chance of winning by switching given we chose door 1 = 2/3). Step 2 strengthens this to a conditional solution (B: chance of winning given we chose door 1 and the host opened door 3 = 2/3). Statements A and B are different. They mean something different. Don't you realise that?   