Why?

I've been wondering when Luna was going to start saying "Why?"  I'd noticed that some other kids about her age were saying it, so I figured it was coming soon.  But "Why" isn't exactly something you can explain to a two year old.  All the words you would use to explain what it means, like "reason" she doesn't understand either.  I mean, "why" is "why!"  We certainly tell her why she can't do things, but she had never made the connection.

So today Luna was in the bath, and the water was running, so I shut it off.  And she said "Why?"  Just why!  SI I said "why?"  And she said "Why stop water Mommy?"

SO here we go!  Now she's asked me "why" about 15 things already!