A curious observation is that primes occur in twins with a surprising regularity. For example: 11 and 13; 17 and 19; 29 and 31; 41 and 43; 59 and 61. Just as with single primes, the frequency of twin primes decreases as one gets to larger numbers. But do they completely fizzle out beyond some very large number? That is the big question. Around a trillion, for instance, only about one in every 28 numbers is a prime.