Three and five are prime numbers — that is, they are divisible only by one and by themselves. So are five and seven. And 11 and 13. And for each of these pairs of prime numbers, the difference is two.

The number 1 is not considered a prime number because it has only 1 positive divisor. Some prime numbers form “twin primes,” like 11 and 13 or 17 and 19. These pairs differ by exactly two. Large prime ...