Compound Interest and the Rule of 72

Compound Interest is an odd thing to figure out. It requires constant surveillance and calculations. Let's say you save $100 in a bank, that offers 10% interest a month. It's an absurd number, but just play along for a bit. Simple Interest would give you 10% of $100 a month, until the percent changes. So every month, you would get $10, which brings you up to $110, $120, $130... Just adding $10 every month. Compound Interest works per month at 10%, just for the example, going up to $110. Then It would take 10% of $110, which gives $11 that month, bringing you up to $121. It would continue to take 10% of the new number, which gives a little bit more than simple interest.