Mastermind

Mastermind is the classic two-player code-breaker distilled into a duel between you and a hidden pattern: a secret code of four colours drawn from six, repeats allowed, and ten guesses to crack it. Each guess comes back not as a yes or no but as pegs — a black peg for every colour that is right and in the right spot, a white peg for a colour that belongs in the code but sits in the wrong place — and reading those pegs correctly is the entire skill. The disciplined way to solve it is to change just one thing between guesses so each new clue isolates a single fact, and to keep black and white pegs mentally separate, since black nails a position while white only confirms a colour is somewhere in play. Beginners forget that colours can repeat, so a stingy peg count never fully rules a colour out. Treat it as elimination first — establish which colours are in before you fret over exactly where they go. It plays free in the browser on any device, no download required.