Q: It doesn't feel like a "modal interchange" or anything fancy like that because the melody will still feel very much like plain old C major. And sometimes too you'll even have a C7 (particularly before going to the F), but it doesn't feel like you're in C mixolydian or anything like that. 
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A: that one you just mentioned reminds me of a door handle that turns sideways when the door opens. even though the handle is normally not in that position, the sideways position IS actually a normal position for the purpose of opening the door. not only normal, but powerful. a real door opener, that C7 (to F)