Rather than trying to explain what a coded message with a function table is, I thought I would show you one:
Fill in the function table at the top, then use the letters to solve the coded message.
If you want more puzzles like this one, I have a freebie with six of them. Each answer is a non-religious holiday song, and I have provided an answer key.
My students beg for math problems like these. I hope yours will too.