Posting Puzzle
A little puzzle about sorting envelopes and stamps. Pair them up to deliver, mix them up to return.
How to play
Pushing a tray adds to both rows at once. Matching pairs go in the mailbox, a mismatch comes back. That is the whole rule.
- 1 / 2 / 3: push a tray
- Z or Backspace: undo
- R: reset
- ← / →: change stage
About the puzzles
Mr. Post is named after Emil Post, and every stage here is an instance of Post's Correspondence Problem. Twelve in all: the first six are meant to be solved by hand, the last six are hard instances borrowed from research and are not. A small celebration of PCP[3,4] finally being settled.
An earlier version: mame.github.io/posting-puzzle. Font: Special Elite. Images made with ChatGPT and Gemini, edited by hand.