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.

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I did that too when my niece had a copy of... I forget the book, but it was a modern version of one of those old-fashioned activity books, and it had a substitution cipher in it.
So I sent her a note in that cipher, saying "If you can read this, I owe you a Mars bar" and left the promised chocolate in her grandparents' fridge for safekeeping.
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I really, really like her.
So I sent her a note in that cipher, saying "If you can read this, I owe you a Mars bar" and left the promised chocolate in her grandparents' fridge for safekeeping.
Aw! That's lovely.