Okay, here's the thing. I live in a building with 4 floors, I am on the second floor. Whenever my upstair neighbor runs the garbage disposal my sink starts to back up and fill with water. Why is this? are my pipes clogged? I want to call a plumber, but since I am in a condo, if the problem is common to both units, then I am not the responsible party as far as repair bill. So what could be causing this back wash debacle?