Calvin and Hobbes by Bill Watterson for August 06, 1993
Transcript:
Calvin: Now this is supposed to look like a coded message from me to you, but we'll leave it for Susie to find. Obviously, the code will have to be easy to break, so she can read the disinformation we're giving her. Hobbes: How about if we write backwards? Calvin: Yeah, that's good! Letter: Dear Hobbes, (backwards). Calvin: Gosh, I hope Susie's not too dumb to figure this out. Hobbes: Cracking codes is second nature to cool spies like us.
That says “Dear Hobbes”