> ... for x in range(2, n): > ... if n % x == 0:( I don't understand the n%x It means find the remainder of an integer division. Thus 7 divided by 3 is 2 remainder 1 So, 7/3 => 2 7%3 => 1 Thus n%x == 0 is true when the number held in n is exactlty divisible by the number in x. And thus n cannot be a prime number... Alan G.