Why €10,000 doesn't become €25,000 after 30 years.
With compound interest you earn interest on interest already credited. €10,000 at 5% grows to €25,000 after 30 years without compounding. With compounding it reaches about €43,200, because each interest payment enlarges the capital that earns interest next year. The curve gets steeper every year.
Useful: The earlier you start, the more of the work time does instead of you.
Source: de.wikipedia.org