The decimal system, which is based on the number 10, is the number system used most in the world. Other number systems you are familiar with are binary numbers, which are based on zero and 1 and are used in computers, and time, which is divided (mostly) into units that are multiples of 60 -- there are 60 seconds in a minute and 60 minutes in an hour, for example. Since the decimal system and powers of 10 are so important in science, I'll talk a bit about them here.