I've been moving with the bodies that move to a different sound

I just taught my niece a simple substitution cipher and within ten minutes she had correctly deciphered a short message I had written to her. I could watch her make the connections, exclaiming as she recognized individual encrypted letters without having to refer to the key. Then she drew a picture at the bottom of the page to make it clear she had not just copied out but understood. Morse code, Playfair, one-time pads, here we come.