My cats have figured out how to open doors. The first time they did it, a couple of weeks ago, I thought it might have been an accident (and it probably was). However, they let themselves in yesterday morning and again this morning, so I guess they've finally figured out the secret. Fortunately, so far they only seem to let themselves in in the morning after we're already up, so we don't have to start locking the door. I guess they're just really eager for breakfast.