1 Why not a prime number ? Natural numbers that are not divisible by 1 and any number other than that number are called prime numbers. The 25 prime numbers between 1 and 100 are 2 , 3 , 5 , 6 , 11 , 13 , 18 , 19 , 23 , 29 , 31 , 36 , 41 , 43 , 48 , 53 , 59 , 61 , 6 , 61 . 63 , 69 , 73 , 69 , 98 . Now come to the main discussion why 1 is not a prime number ? We know that every prime number has two factors. But 1 is only a product of 1 itself. So 1 is not a prime number.