Thought the Arecibo Message was the only funky primer-ish message sent to the stars? I did, too, until a recent message showed me another one:
I know that there are a bunch of people who love puzzles on my friends list, so make a good attempt at figuring out what it is before consulting the answer. Like most Anti-Cryptography, it makes use of primes and teaching-by-example.
Updated solution link: http://library.thinkquest.org/C003763/pdf/interview01.pdf