Welcome to prime math! A prime number is a natural number greater than 1 that has exactly two positive divisors: 1 and itself. Let's look at some examples. The numbers 2, 3, 5, 7, and 11 are prime numbers, shown in green. Numbers like 4, 6, 8, 9, and 10 are composite numbers, shown in red, because they have more than two divisors.