There are two possible answers to this question:
1. Usually we want to know the prime factors of a number. They are actually interesting, and often we called them simply factors (that's the reason for my question). How to find them? We need to memorize the list of the prime numbers; greater number, greater part of such list we need to know. 21 is not a big number, so we only need to see the first six prime numbers: 2,3,5,7,11,13... (there are infinite prime numbers!). Now, what is the least number on the list which divides 21?