You sure 1 isn't prime? It can only be divided by itself and, well, 1.
The ancient Greeks regarded 1 as a prime.
Today it is not.
Mainly because a lot of important theorems would be false
if 1 were a prime.
Yeah, one's not prime.
A prime number, by definition, is a number that can only be divided by two numerals, itself, and one. One can only be divided by one, which means that there aren't two digits, so it falls out.
I find this definition especially irritating, because "prime" is a derivate from Latin meaning "one" or "first". So it's aesthetically displeasing, from a linguistic standpoint, that the number 1 should NOT be "prime". But it's not, according to the definition.