I have a nut for you guys to crack

I just wanted to speculate around the subject of making it look like an object is behind something. By this I mean like having a stool behind a window when there actually is no stool. Not the erasing text kind of thing that looks nice and classy in it's own way, but I mean the actual getting something to look like it's behind something. Like a chair, a person. How does one do that?