So far, we've treated flows as independent: the tap runs, the drain opens. But in most real systems, the output of one process becomes the input of another — and sometimes feeds back into itself.

This is feedback — the defining feature of systems. And the most dramatic kind is the reinforcing loop.

A reinforcing loop amplifies whatever is happening. If something is growing, a reinforcing loop makes it grow faster. If something is declining, a reinforcing loop makes it decline faster.

The classic example: compound interest. You deposit money. It earns interest. The interest is added to the balance. Now you earn interest on a larger balance. Which earns more interest. Which grows the balance further.

The same structure, in reverse: debt spirals. You owe money. Interest accrues. The interest is added to the debt. Now you owe interest on a larger debt. Which accrues more interest. Which grows the debt further.