Why do numbers become a repeating decimal when divided by 9?

Numbers become a repeating decimal when dividing by ANY whole number, except when (if the division, expressed as a fraction, is in simplest terms) the denominator has no prime factors other than 2 and 5.The reason for this is related to the fact that our decimal system is base 10, and 10 is made up of the factors 2 and 5. Any denominator with other factors cannot be exactly expressed as a terminating decimal. That includes denominators 3, 6, 7, 9, 11, 12, 13, 14, 15, 17, 18, 19, 21, etc.