The Next Generation Science Standards ask students to analyze a system by defining its boundaries and initial conditions, as well as its inputs and outputs. Students then use various types of models to map out or simulate flows (of energy, matter, information, money, etc.) within and between systems.

One such model that is often used is a Stock and Flow Diagram. While causal loop diagrams help students visualize closed loop cause-and-effect relationships, stock and flow diagrams represent the interconnected accumulations (or stocks) and inputs/output (or flows) within a dynamic system. Stock/flow diagrams are quantifiable:  the population of wolves or moose at a given time might be a stock. The amount of something in a stock changes by flows, which are measured over a certain time, e.g., the number of wolves born per month. By including numbers, stock and flow diagrams enable us to test how one variable interacts with another variable, and determine what behavior that produces. A quantified stock and flow model becomes a natural transition to computer models. 

Read this essay to learn more about stocks and flows.