Heavy doors, "no". And Mimi doesn't open anything. She doesn't need to because we wait on her hand and foot! Butter though, has figured out that the catch on the glass storm door on the front door is an easy one. He bangs against it over and over until it opens just enough for him to slide outside when I have the main door open. I usually hear him fast enough to come running and open it for him. I would hate for that striped tail to get caught in the slamming door! Butter's other habit is to "find" me in whatever room I'm in (usually my bedroom) and bang on the window screen to come inside. His favorite one is the bathroom window where the screen is loose. Then I just pop the window open and he comes in that way!