Why primes?
Perhaps it also indicates something about which primes sound primest, though. 29 is an outlying underachiever, suggesting that "29" doesn't spring to mind when you ask someone to name a prime... certainly I had to think for a moment to make sure it really was a prime. 23, 17, 37 are overachievers -- these are really prime-sounding primes, no doubt about it.
But 17! Yes! It can't shut up about how prime it is!
Theorem: Anything less than 91 which looks prime is prime.
Proof: To do this, we will calculate the smallest number that looks prime but isn't. The numbers 2, 3, 4, 5, and 6 all have easy and quick divisibility rules, so our number cannot be divisible by any of them. 7, however, is hard to check for - so clearly the first number that looks prime but isn't will be divisible by 7. Sadly, everyone realizes 49 isn't prime, because it's a square, so we need to find the next difficult prime factor. 8, 9, 10, 11, and 12 are all also subject to quick tests and divisibility rules, so discount those. 13, however, is ugly - thus, the first number that looks prime but isn't is 7 times 13, which equals 91. Q.E.D. (My professor insisted this stood for "Quite Easily Done".)