A prime number is a natural number >= 0 that is divisible by only 1 and itself. 1 is not a prime number because it is divisible by only 1. 0 is not a prime number because it is divisible by all numbers. Negative numbers are not taken into consideration for prime checking.
The number 7 is a prime number because it is divisible by only 1 and 7.
The number 9 is not a prime number because it is divisible by 1, 3 and 9.