decode is a free, no-signup take on the classic code-breaking puzzle — the pen-and-paper game long known as bulls and cows, and later as a boxed game with coloured pegs. You crack a hidden 4-colour code by deduction: each guess earns clues (🟩 right colour & place, 🟨 right colour wrong place) and you narrow it down.
Build a guess from six colours across four slots, hit Guess, read the 🟩/🟨 clues, and deduce the code within eight tries. Full rules and strategy are on the how-to-play page.