The math trick that turns $10/day into $1.8 million

Compound interest is when your earnings generate their own earnings. Einstein reportedly called it the "eighth wonder of the world."

Here's the magic: invest $10/day ($3,650/year) at a 10% average annual return (roughly the S&P 500's historical average). After 40 years, you'd have about $1.8 million. But here's the kicker — you only invested $146,000 of your own money. The other $1.65 million came from compounding.

The Rule of 72 makes this easy to estimate: divide 72 by your annual return rate to see how many years it takes to double your money. At 10%, your money doubles every 7.2 years. At 7%, every 10.3 years. The key lesson? Start early. Time is the most powerful variable in the compounding equation.