Tough second interview- be ready for questions that are there to trick you a bit. For example, one was to consider a refrigeration unit that is infinitely efficient and left open inside of a room that is perfectly insulated- will the temperature change? Talk through your answers and ask clarifying questions clearly. I will note that with these very theoretical questions, you need some assumptions to solve them and would think that you need to ask about potential assumptions, but my interview seemingly really did not like that.