Terry Tao was on The Colbert Report the other day - I'm not sure why. Tao is one of the most distinguished living mathematicians as well as a famous prodigy (he scored a 760 on the Math portion of the SAT at age eight). He and Colbert discussed twin primes, cousin primes, and sexy primes though not much light was shed.
At one point during the discussion of primes, after a few twins had been mentioned (5-7, 11-13) Tao said the number twenty-seven, and stopped. He didn't say anything about the number twenty-seven, which of course is not prime, but the mere mention reminded me of the legend of the Grothendieck "prime", fifty-seven. Grothendieck, the story goes, had been teaching a class where he had been talking about "consider a prime" when a student asked for an example. Well, OK, the story goes, Grothendieck picked 57.
(Which, of course, is 3 x 19.)