Answer by
Diddum (348)

They are consecutive prime numbers whose difference is two, like (3,5), (5,7), (11,13), (17,19),
(29,31) and so on. A prime number is a number greater than 1 which is divisible only by 1 and by the number itself. There are infinite prime numbers, and the first ones are 2, 3, 5, 7, 11, 13, 17,...