"Why?"

One of Rachel's newest things is to ask "why?" every time I tell her to do something.  The teacher commented, recently' that she was asking "why?" at school too.  She has the perfect inflection with it and really seems to be asking for an explanation, but she only says it once.  I don't want to sell her short, but I am thinking that Rachel is not actually asking for me to explain myself, she is just responding to me with "why?" because she has noticed that her sisters do it ALL.  THE.  TIME.  I think that was one of my biggest fails as a parent......giving the older girls explanations for why they should do something.  I am paying the price now.  Sooooo, poor little Rachel gets no explanations.  I say, "Go wash your hands."  and she says, "Why?" at which point I just tell her again to go was her hands.  She will usually go ahead at that point and do what I have told her to do.  Now if I could just get that to work with the older two!