I think it can remember 2 questions because I referred to my first question in my second question and it answered. First I asked about the invisibility cloak then I asked if I can use it to sneak into girls dressing room.

good insight! I use a lookback window of one response, because it gets pretty expensive to do more.

Why is it expensive ? you can't just paste the plain text of the whole interaction as input ?

OpenAI charges per token - so whatever I send as context is counted. This easily 2x, 3x, 5x's the costs depending on the length of the chat.