Substitution

Miss 10 has been writing things in her own "language", which looks very impressive.
Me: "is it just straight letter substitution?"
Her (without pausing): "yes"
Me: "do you know what substitution is?"
Her (long pause): "no ..."
After explaining to her what substitution meant and then taking a few pages of her coded writing away, we cracked it in about 2 hours - made somewhat harder by spelling errors, made-up words, wrong letters and several very similar symbols.
I can now write her notes in her own "language" and leave them for her to find.