In the language of system dynamics, important system variables are represented as stocks, flows, and feedback loops. Stocks are the accumulations points in a system. Simple examples of stocks are water that accumulates in a bathtub, accumulations of product inventory, or money that accumulates in a bank account. are measured in units such as gallons, items, or dollars. Flows are variables that add to or subtract from stocks. The flow into the bathtub is water flowing through the faucet while the outflow is water flowing down the drain. The inflow into inventory is production while the outflow is shipments. Flows are measured in units per time such as gallons per minute, items per month, or dollars per year. Interacting stocks and flows generate the dynamic behavior of metrics such as MC and cost.