Compounding

Put a single grain of rice on the first square of a chessboard, two on the next, four on the next, and keep doubling. By the last of the 64 squares you owe more rice than has ever been grown in human history — a heap larger than Mount Everest. Nothing about the rule felt dangerous; you only ever doubled. That gap — between a rule that sounds gentle and a result that is monstrous — is the whole reason compounding is one of the most important and most under-felt mental models you will ever own.

Compounding is what happens when growth feeds on its own output: each period’s gain joins the pile and itself starts earning. Money makes money, and that money makes money. The line it traces isn’t a ramp; it’s a curve that lies almost flat for a long, boring while and then erupts — the shape people call “slowly, then suddenly.” Your brain, which quietly expects things to add up in straight lines, is built to misjudge it at exactly the moment it matters most. That single blind spot explains why people give up on savings, habits, and skills right before the payoff, and why others get buried by debt or technical shortcuts that compounded against them in the dark.

This course assumes you know nothing but what a mental model is, the idea you met in the very first course. We’ll start by feeling the difference between adding and multiplying and why human intuition is wired for the first; pin down the actual formula and the Rule of 72 so you can do doubling-time math in your head; watch what breaks the chain — a single big loss, a withdrawal, an interruption — and why one −50% year can erase a decade; and finally see that compounding is not a money trick at all but a general engine that runs on skills, relationships, reputation, knowledge, and — pointed the wrong way — on debt and decay. You’ll drive an interactive curve that bends in front of you and race two savers to prove the most counter-intuitive result in personal finance: that when you start usually beats how much you put in. By the end you’ll read a flat-looking early stretch not as failure but as the part of the curve right before it takes off.