Some time ago, I wrote about two types of numbers. Now Mike Russell of Pivotal Writing has created a nice introduction to stocks and flows that includes a quiz, courtesy of Dr. John Sterman of MIT. Check it out, and see how well you do.
While you're on this subject, check out my More on two types of numbers from 2005. While the concept is still as valid as it was then, the view of the example I chose sounds so dated today.