The online computer game “Is this prime?” tests a player’s knowledge of prime numbers—and just surpassed 2,999,999 attempts. Give it a whirl.

9 is not a prime number. It can be divided by 3 as well as 1 and 9. The prime numbers below 20 are: 2, 3, 5, 7, 11, 13, 17, 19. Don't forget: the number 1 is not thought of as a prime number.