The first thing you learn as a ghost is how the stairs creak. Not just which stairs creak, but the notes and melodies they play. Initially, you notice what the order means, whether someone is rising or descending. Then you start to recognize feet as they pass, how they pause, and where they hesitate. You learn the sounds you can make and how to make no sound altogether. Sometimes, you learn to mimic people, and sometimes you learn to sound like no one.