Compound interest

Simple interest pays you on your original amount. Compound interest pays you on your original amount plus every gain it has already made. Over one year the difference is trivial. Over thirty it is the difference between doubling your money and multiplying it by ten.

Compounding is why time in the market matters more than timing it, and why the boring advice to start early is genuinely the highest-leverage financial decision most people ever make. It is also why high-interest debt is so destructive - the same mechanism, running against you.

Watch where the money comes from. Paying $500 a month into something returning 7% gives about $86,000 after ten years, of which $60,000 is your own contributions. Leave it thirty years and it is roughly $610,000 against $180,000 contributed - and more than half of that total arrives in the final ten years alone. Nothing about your saving changed; the base it worked on did.

The same mechanism runs backwards on debt, faster, because the rates are higher. A $6,000 credit card balance at 22% costs around $1,300 a year to simply keep existing. Clearing it is a guaranteed 22% return, which no investment can honestly promise, and it is why paying down expensive debt comes before investing in almost every case.