Stocks are also known as levels, accumulations, or state variables. Stocks change their values by accumulating or integrating rates. This means that the values of stocks change continuously over time even when the rates are changing discontinuously. Rates, also known as flows, change the value of stocks. The value of a rate is not dependent on previous values of that rate; instead the stocks in a system, along with exogenous influences, determine the values of rates. Intermediate concepts or calculations are known as auxiliaries and, like rates, can change immediately in response to changes in stocks or exogenous influences.
When constructing a Stock and Rate diagram, consider what variables accumulate over a period of time. Another way to think about this: if Time slowed down to zero for your system, what variables would still be nonzero? For example, in the system where you pour water into a glass, the water contained in the glass is the Stock. If you froze time, the pouring (a Rate) would stop, but you would still see a quantity of water in the glass (a Stock). Once you know what levels you need, enter them first and then connect the rates and auxiliaries. Model building tends to be iterative. Don't try to get everything right the first time; you can always change things later on.
For the problem we are working here the levels are Potential Customers and Customers.
| Ø | Select the Stock tool  and click once on the diagram. Type in Potential Customers and press Enter. |
| Ø | With the Stock tool still selected, click on the diagram approximately 3 inches (7 cm) to the right of Potential Customers, type in Customers, and press Enter. |
