I thought I had something profound to say...

I guess it was just indigestion.

But anyway, I think there are 3 phases to understanding something.

Starting from the point of complete non-understanding from which you just have a negative understanding of what is going on about the concept you are trying to understand.  In fact, people around you are losing I.Q. points whenever you look at them...

1.  I think I have a rough clue about what is going on but don't ask me anything more complicated than what you just showed me.

2.  I have a good grasp of the concept/process and one deviation from the normal won't send me in to a crying fit.

3.  I know this well enough to explain it to other people stuck at zero and can actually predict where it might fail or make improvements on it.

4.  You are Albert Einstein and just made the idea out of duct tape and bailing wire.

In fact, I believe one of the best tests of understanding is trying to explain it to my wife or Lucas.  Sometimes it is easier to explain it to my two year old (Lucas) than wife but I digress.