if one day we have a machine which can records every atoms' position and kind,and breake down the object into atoms,then rebuild the object use the record before. is the object after such process still the same object before it? if we have a clock,and we tear down it. is it still a clock? of course not. but if it is broken,and we just change one part of it, is it the same clock? if it is ,which part should we change ,then it's not?