THE NUMBER 2 IS A COMMON FACTOR OF ALL EVEN NUMBERS.
In order for number A to be a factor of number B, number B should be divisible by number A. The result is ONLY an integer.
Obviously, no odd number can be evenly divided by 2. Remember, decimals don't count!! If you divide 7 by 2 you get 3.5, which is not an integer. So only even ones are divisible by 2.