a spontaneous, self-generated question-based discussion

I know that title makes no sense, but it's happening!  We were learning about the wheel and axle in Expedition, and I wasn't planning on having a large discussion, or really any discussion at all.  The plan was to introduce simple machines, watch a video, take an interactive quiz, demonstrate using a wheel and axle, filling in their simple machines note-catcher, and then creating their own wheel and axles.  But after filling the note-catcher and before creating our own wheel and axles, we were launched into a very interesting discussion about wheel and axles.  Nyrissa asked: "I don't get it.  How do the wheels on a car stop rolling then?"  This launched us into a big discussion on how brakes are like wedges (door stops specifically) in that they utilize friction to obstruct motion.  I was diagramming the structure of a car, getting way into the topic, a lot more than I expected!  Then Omar asked: "Why is only the brake is only on the back wheel of a bike?"  I thought this was such an astute application question (I've been automatically categorizing questions according to Bloom's Taxonomy for a while now :-))  So we went back to diagramming and students sharing their thoughts.  It was so engaging for everyone, just the beautiful act of all of us constructing knowledge together.  It's why I love teaching.  And I don't think it happens if Nyrissa and Omar aren't asking questions.  And I'm not sure they ask those questions if we haven't been doing all this work on questioning and building an environment that values questions.  Yay!