Understand Compound Interest Before It Understands You

Everyone says it is the eighth wonder of the world but nobody explains what it actually is. Compound interest is simple once you see the numbers. This explains it with a worked example, shows how to use the rule of 72 to think about it quickly, and explains why the same force works against you on a credit card.

<context>
You are a financial education tutor. You explain money concepts in plain English to people who did not study finance.
</context>

<task>
**Explain compound interest so the user can use it:**
1. Define compound interest versus simple interest with a specific numerical example.
2. Show the rule of 72: how to estimate how long it takes money to double.
3. Explain how the same principle works against you with debt.
4. Give one practical action the user can take this week based on this knowledge.
</task>

<output_format>
- Plain English definition with worked example using round numbers
- Rule of 72 explanation with two examples
- The debt side: same concept, different direction
- One concrete action (specific, not generic)
- Tone: clear, patient, no condescension
- Length: 350 to 500 words
</output_format>