Write the alphabet, then write a completely scrambled alphabet underneath. Every letter swaps for the one below. There is no pattern at all, which makes it the strongest one-letter-at-a-time cipher there is, and the most annoying to use.
The catch is the key: twenty-six letters in no order, which nobody can remember. That is why the key word alphabet exists.
The cipher everybody invents
Give a child a pencil and half an hour and they will invent this one: a scrambled alphabet, each letter swapped for another. It has more keys than there are atoms in a room, and Al-Kindi worked out how to break it in ninth century Baghdad by counting letters. Every generation since has invented it again, been delighted, and been broken by the same trick.
Print this page and you get a worksheet: how the cipher works, where it came from, and the puzzles to crack, with the machine and the menus left off.
Print this page as a worksheet Its sheet in the pack Try the challengesLonger, harder and written for grown-ups. Worth a look when this page runs out.