Too Smart

David is getting too smart for us and his own good.  We have recently entered the "why" phase of growing up.  Why? is incessantly the question that never gets a good enough answer.  One day after getting tired of answering the question Dustin decided to turn the question back on David to get him to think about it.  I will tell you that this tactic worked THREE times - that's it!  David caught on and now turns it back on us.  Here is an example:
Me:  David you need to clean up your toys.
David:  Why?
Me:  Because it's good to clean up your toys when you're done playing with them so they don't get broken.
David:  Why?
Me:  Why do you think you should clean them up?
David:  Why do you think you should clean them up?
We usually stop there because what else am I supposed to do?  I must say I have heard parents complain about this "why?" phase but I never really understood just how bad it could be.  At least he is learning something, right?