When you save money in a normal account, it just sits there. Compound interest is different — it's when the interest you earn also starts earning interest. Think of it like a snowball rolling downhill: it starts small, but the further it rolls, the faster it grows, because it's picking up more snow with every turn. In investing, your money works the same way. The longer you leave it, the faster the growth accelerates — which is why starting early matters more than starting big. Use the calculator below to see exactly how your own numbers play out over time.