IN ONE SENTENCE: Compound interest means you earn interest on your original money and also on the interest you already earned. AN ANALOGY: A snowball rolling downhill picks up more snow as it gets bigger, so it grows faster the longer it rolls. Where it breaks: A snowball keeps growing, but real interest rates change, and money can also lose value or be lost. STEP BY STEP: 1. You put in a starting amount, called the principal (the money you begin with). 2. After a period, such as a year, the bank adds interest based on a rate, like 5%. 3. That interest becomes part of your balance. 4. Next period, interest is calculated on the new, larger balance. 5. Repeat, and the amount added each period keeps getting bigger. A CONCRETE EXAMPLE: Start with $1,000 at 5% interest per year. Year 1: 5% of $1,000 is $50, so you have $1,050. Year 2: 5% of $1,050 is $52.50, so you have $1,102.50. Year 3: 5% of $1,102.50 is $55.13 (rounded), so you have $1,157.63. With simple interest (always 5% of the original $1,000, which is $50 a year), you'd have $1,150 after 3 years. After 10 years, compound interest gives about $1,628.89 and simple interest gives $1,500. COMMON MISCONCEPTION: Many people think 5% a year means 5% of the starting amount every year. That's simple interest. With compound interest, each year's 5% is taken from a bigger balance. CHECK YOURSELF: 1. You put $200 at 10% per year for 2 years, compounded yearly. How much do you have? Answer: $242. Year 1: $200 × 1.10 = $220. Year 2: $220 × 1.10 = $242. 2. Over a long time, which grows more, simple or compound interest at the same rate? Answer: Compound, because the interest keeps being added to the balance. 3. In your own words, why does the amount added each year get bigger? Answer: Because the interest is calculated on a balance that now includes earlier interest. WHERE TO GO NEXT: How often interest is compounded (yearly, monthly, daily), and the difference between an interest rate and an annual percentage yield. VERIFY THIS: This explains the idea in general terms, not advice for your own money. Real accounts compound on different schedules and charge fees, so check the terms of a specific account or ask a qualified professional before deciding.