Actually, that's a good question. Logically, you are right, but \(1\) is a special number that is a factor of all of integer numbers, since any number is divisible by \(1.\) So any two random integer numbers have a common factor of \(1.\) That is why when mathematicians say that "two numbers doesn't have common factors", they mean that those two numbers don't have any common factors except for \(1.\)