Roadkill did it with the rotson <| sep |> Thank you . I know it was rough because I rushed it before I took a shower one day . I was gonna go to ulta Monday and look for lighter shades of foundation so I could practice . I wanted to get cheap stuff until I got good enough for the hq stuff . I appreciate the feedback <| sep |> It ' s very easy to tell that something isn ' t a prime number . As you said in your example , you can easily show that 6 isn ' t a prime number by showing it divides by 2 . Or by showing that it divides by 3 . You don ' t have to find every factor , you just need to find one factor and you ' ve proved the number isn ' t prime . Proving something is prime is much harder . In your example , you said But 7 can only be found by adding 1111111 . But how do you know that ? The only way you can prove it is by saying that 2 isn ' t a factor of 7 , and 3 isn ' t a factor of 7 , and 4 isn ' t a factor of 7 . You need to repeat this for every number right the way from 2 up to your number actually , up to half of your number . And for a number like 7 , that doesn ' t take too long . But for a number that ' s 10 digits long , it takes a while . And for a number that ' s 20 digits long , even the most powerful computers will struggle to do it in a sensible timeframe . And that ' s why it ' s so hard .