Why?

I propose that a child has a limit on the number of times a kid can use a single word sentence in the form of a question.  I love that my daughter is a sponge and that she questions everything around her.  But I don't like the fact that everything I say is followed by one single word....why?  Well, I take that back.  I'm ok if she uses it when we're talking about something random.  What I'm not ok with is that she asks why after I give her an instruction, or tell her not to do something, or tell her she can't have something...see a pattern yet?  :) 

The following is an actual conversation I had with...well...basically me, myself, and I, because the only other words used during the conversation were "why?". 

Me:  Zoe, Ethan, let's go. 

Zoe:  Why?

Me:  Because we need to go to Walgreens to get your brother some medicine.

Zoe:  Why?

Me:  Because he ran out of his allergy medicine and needs some more.

Zoe:  Why? 

Me:  Because otherwise, he'll get sick.

Zoe:  Why?

Me:  You sure ask a lot of why questions.  Let's not ask that after everything that I say, ok? 

Ethan (now putting in his two cents):  Why?

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So...who second's my proposal??