I find that some times the flute becomes suddenly easy and smooth to blow even tho I might have been struggling half an hour previously. Since Doug posted his dimple topic, I started to peer insude my flute when this happened. And this is what I saw:

A perfect layer of tiny droplets, mop the flute and unless my imagination is deceiving me, a similar tone requires more effort to achieve. It’s the second octave that changes most. Very odd.