As sweet and endearing as the "why?" phenomenon is most of the time, there comes a time when you don't want to answer it anymore and she's really just asking it to stall going to bed or getting in her car seat or whatever it is you are asking her to do. I have found me repeating the age-old "because I said so!" just like my parents did over and over to me. I try to limit it's use, but man it comes in handy sometimes!
Another favorite, especially when she is asking about nature or why the world is the way it is, and she has surpassed general knowledge or is getting deeper than I care to go at age three, the response "because that's the way God made it" works really well. I know I'll expand her knowledge later, but trying to explain why boys have tee-tees and girls have tu-tas really only deserves one response at this point: because that's how God made it.
I wonder how long this new stage is going to last? Fun, Fun, Fun!!