system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 why the number 1 is not a prime number?
gpt
The real reason is that if you exclude 1, then you end up with a very useful property that all numbers are made up of a unique group of prime numbers that act like a finger print. This concept is so valuable that it is called the Fundamental Theorem of Arithmetic and it is the reason why we care about prime numbers. This property is also the basis for modern Cryptography.

So the number 60 is 2 \* 2 \* 3 \* 5, so the group of numbers 2, 2, 3, and 5 describe 60 uniquely. However, if 1 is a prime number, than 1 \* 2 \* 2 \* 3 \* 5 would also describe 60, as well as 1 \* 1 \* 2 \* 2 \* 3 \* 5 and so on.

By the way, everyone saying that it's because a prime number has exactly two divisors is wrong. We don't really care about the number of divisors, we care about what we can do with primes, which is break up any number into primes. Source: Degree in math.