Backward Facing Step visualization using Rubber Dust

These days I'm working on the CFD of backward facing step. And suddenly, while I was in class, I remembered a game I have been playing since 4th grade at primary school.

I knew that by blowing I could blow away the rubber dust, as long as I've known myself. But one day I had realized that, when I blew air over a think notebook, the rubber dust on it would fly away, whereas the rubber dust just behind the notebook would come towards me not away!

I've been examining this "weird phenomenon" whenever I was to blow rubber dust or automatic pencil leads...

Lovely thing, that now I can understand it's because of the circulation behind the backward facing step. And the game I've played for that long is just a simple way of visualizing flow structures, wall-shear in this case.