What is the rule of 72 for doubling your money?

A simple way to figure out how long it will take to double your investment funds if to take the number 72 and divide it by your expected return. For example if you expect an 8% return (The average return of stocks over time) you would divide 72 by 8 and it would take you 9 years to double your money. This is why you cannot leave your investment funds in the bank. If you are receiving 1% interest it will take you 72 years to double your money!!!!!!