The first Prime number is 2. If you're wondering why 1 is not prime, well, probably because you cannot break a number down if you divide it by 1. When it comes to prime factorization, if you bring 1 into it, the number you start with will stay the same (e.g., If you star a "prime factorization tree" for 100 and start off with 1, you will still have 100. You do not "grow" another "branch.")