Compound Interest & The Rule Of 72

Well, first off, Compound Interest is the act of interest sort of "piling up" over time. For example, should you deposit $100 in the bank, with a 10% interest rate. Next year you'll have $110 in the bank right? Now the following year your investment will be $121 because your interest rate gave you 10% of the previous year's interest, so it was COMPOUNDED. Now that this has been explained, the next thing I would like to get into is the Rule of 72. At first its principles were a bit unclear to me, but now I understand them. The Rule of 72 is used to determine the time in which your deposit will be double of the amount you put in. An example of this would be, if you put in a $100 deposit with an interest rate of 9% a year. Using the rule of 72, [72/9=8] it would take 8 years for the $100 to become $200. That's about it.