Read this. I first read about this arithmetic in Charles Givens' More Wealth Without Risk and I've cited it often, but this example (taken from a book by Burton Malkiel) spells out the math in specific terms. The knowledge startled me when I first read it, and the illustration startles me today — but thankfully, I've begun saving for retirement during the intervening years.
We don't teach personal finance in school. This is absolutely essential information; and I don't know about you, but it was news to me. The principle may be obvious, but the details and the magnitude constitute a revelation of their own. People need to know. Pass it on.