She walked up to me and said "B-nothing-nothing-G." I looked up from my neurotransmitter research, "What, honey?" "B-nothing-nothing-G! That is 2007, mom." I stare at her blankly and ask "who taught you that?" "I was just thinking, and decided to try to number the letters of the alphabet, and then I used the alphabet for numbers. It is fun." She ran off to play fairies.
Mental note: February 2007, 5-year-old invents the number-letter substitution cipher for a lark. I am screwed.
UPDATE: She went from the number cipher rapidly to writing in Morse code. I figured it out! She was avoiding having to write in upper and lower case letters. I told her she could use all caps if she was more comfortable and problem solved. She still does give me notes in code here and there.