The page and the mirror
The top panel is your paper. The bottom one is what a mirror shows. Press write it backwards and the paper flips: now the paper looks wrong and the mirror looks right.
Leonardo da Vinci wrote thousands of pages like this. People love to say it was to keep his ideas secret, but he was left-handed, and writing right to left with a quill stops you dragging your hand through wet ink. A secret that is really a smudge.
It is the weakest hiding on this site: anyone who holds it up to a window has broken it. But it is the fastest to do, and a child can write a whole note in it in ten minutes.
Hiding that depends on where you stand
Mirror writing belongs to a family: messages that are only readable from one position. Anamorphic paintings that look like a smear until you stand at the side of the room. Text stretched flat on a road so it reads correctly to a driver. The word AMBULANCE reversed across the front of a van, so that it comes out the right way round in your mirror as it comes up behind you.
Nothing is encrypted and nothing is concealed. The message simply refuses to work until you are in the right place, which is its own kind of key.
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