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Compound interest means earning interest on your interest. Money grows not in a straight line but in an accelerating curve, because each period’s growth builds on all previous growth. A useful mental shortcut is the Rule of 72: divide 72 by your annual return to estimate how many years it takes money to double. At 8 percent, money doubles roughly every 9 years, which means $10,000 becomes about $20,000, then $40,000, then $80,000 over three decades without you adding a cent.
The most important variable in compounding is not the amount or even the rate; it’s time. Someone who invests steadily in their twenties and stops can end up with more than someone who starts in their forties and invests far more in total, because early dollars get the most doubling periods. This is why ‘start small now’ beats ‘start big later’ almost every time.
Compounding also has a dark twin: it works identically against you in debt. Credit card balances at around 20 percent or more compound viciously, doubling what you owe in under four years if unpaid. That symmetry gives you this course’s core strategy in one sentence: get compound interest working for you (investing early) and stop it working against you (killing high-interest debt fast).