Compound interest
Interest calculated on both your original principal and the interest it has already earned.
Compound interest makes money grow at an accelerating pace, because each round of interest is charged on a larger balance. The longer the horizon, the stronger the effect — which is why it is often called the eighth wonder of the world.
Example100 leva at 7% a year grows to about 197 leva in 10 years and about 387 leva in 20 — the second decade adds more than the first.