Prime number - Conditional expression in For loop

Hi,

This program is created to find the prime numbers between 2 and 20.  
inside the second for loop, the conditional statement is i <= num/2.
why do we use this as our conditional statement?

int factor;
bool isPrime;

for (int num = 2; num < 20; num++)
{
   isPrime = true;
   factor = 0;
   Console.WriteLine("num " + num);
   // see if num is evenly divisible.
   for (int i = 2; i <= num/2; i++)
   {
      Console.WriteLine("i: " + i + " num/2: " + num/2 + " num%2 " + num%i);
      if ((num % i) == 0)
      {
         // num is evenly divisible.  Thus, it is not prime.
         isPrime = false;
         factor = i;
      }
      Console.WriteLine("factor: " + factor);
   }

if (isPrime)
   Console.WriteLine(num + " is prime.");
else
   Console.WriteLine("Largest factor of " + num + " is " + factor);