Level 1 — Money Basics · 6 分钟

Interest: The Same Force, Both Directions

Interest grows savings and grows debt with identical math — you're always on one side of it.

Interest is rent paid on money. When you save, the bank pays you rent. When you borrow, you pay the lender. The percentage (APR or APY) is the rate; the balance is what it works on; time does the multiplying.

The direction gap is enormous: savings accounts pay a few percent in good years, while credit cards charge 20%+ — which is why $1,000 of card debt at 22% (about $220/year) can never be outrun by $1,000 of savings at 4% (about $40/year). Paying off expensive debt IS earning its rate, guaranteed.

Compounding means interest earns interest. It's slow at first and absurd later — the reason starting small and early beats starting big and someday. Run any numbers you like in the compound calculator; the shape of the curve is the entire lesson.