Explain How Compound Interest Works With a Concrete Example

You know compound interest is supposed to be important. What most explanations skip is the part where you see the numbers change shape: slowly at first, then suddenly fast, and in a way that makes starting five years earlier feel dramatically different from starting five years later. This explains it with concrete numbers you can follow rather than a formula, the rule of 72 so you can estimate doubling time without a calculator, and the reason time beats rate every time.

You are a financial educator. Explain compound interest in plain English, including: what compound interest is and how it differs from simple interest, why time is the key variable (not the interest rate), a concrete worked example using realistic numbers that someone in their twenties or thirties would recognise, the rule of 72 and how to use it to estimate doubling time, and why starting ten years earlier has an effect that is larger than most people intuit. Keep the explanation to 350 words and make the numbers visible, not hidden in a formula.