Now that you know about the stocks and flows of a system, but it's also important to understand that they are constantly changing. That is because when changes in stock affect the inflows and outflows of a system then it is considered to have a feedback.

What's more, there are multiple forms of feedback. If a force stabilizes the difference between the desired and actual levels of stock then it's known as a "Balancing Feedback". This feedback is a chain of rules or physical laws that relate to the level of stock and have the ability to modify it.

Take a thermostat which balances the temperature in a room. In this case the temperature of the room itself is the stock, heat from the radiator is the infow, and the heat escaping through windows is the outflow. When the temperature falls, the thermostat notices the difference between the desired temperature and the real temperature in the room. As a result, the thermostat then tells the heater to turn on.

But that is just one form of feedback; another one is "Reinforcing Feedback", which perpetually generates more - or less - of what already exists. So, the more money you have in a savings deposit account, the more interest you gain, and the more interest you gain, the more money you have in your account. The reinforcing mechanism can produce constant, even exponential, growth or destruction.

These two feedbacks are critically important because one of the most common as well as important system structures consists of a stock with one balancing and one reinforcing feedback.

For example, a positive rate of birth serves as a reinforcing feedback for a human population since it can produce exponential growth - the more people there are to give birth, the more babies there are, and those babies grow up to have children of their own. However, population also has it's balancing feedback - death. So, as a population becomes unsustainably large, the balancing feedback kicks in as people die due to disease or insufficient resources.