This script will detect if a given number is prime by a brute force method. It will start dividing by each integer below the square root of a given number and stop if it detects an operation without a remainder.
5 (which is the square root of 21 plus 1) will be the maximum value checked.
2 is not a multiple of 21
21 is divisible by 3 - 21 is not prime
21 is not prime!