Shift the Wheel
Every letter of a Caesar cipher moves the same distance, so one number opens the whole message — and the message's own shape can hand you that number.
A Caesar cipher wheel, unrolled into two strips of the alphabet. The top strip never moves. The bottom one slides, and when it slides every letter goes with it — the same distance, all at once. There is no control here that moves one letter on its own, because a Caesar cipher cannot do that.
The first message comes with its key, drawn as a row of cells. Turn the wheel until your row is the same length and KHOOR turns into HELLO.
The second message comes with no key at all. Turn until words appear. That already tells you something: nobody had to hand you the key, because there are only 25 keys in the world and you can visit every one of them.
The third message is the point. It also has no key, and you do not need to try all 25. Look at it first. One letter is standing on its own, with a gap on each side — so it must be a whole word, and English has exactly two words that are one letter long: A and I. Press that lone letter and watch 23 of the 25 keys strike themselves out in front of you, without your having tried a single one.
That is the lesson worth keeping: a message can be a secret and still not be safe. The shape of the writing — where the gaps fall, which letters repeat — leaks the key even when nobody says it aloud. Real codes today have so many keys that nobody could ever try them all. A Caesar cipher has 25.
Nothing here can get stuck. The wheel turns both ways and wraps round, and "Start again" is one press.