Break a secret number by reasoning from the clues: each guess returns how many digits are exactly right and how many are right but misplaced.
A hidden code of digits is generated. Enter a full-length guess and the game scores it, then keep deducing until you reveal the number.
Bulls and cows is a pencil-and-paper game old enough that its descendants are more famous than it is. Two players each write a secret number and narrow each other down by guessing — a bull for a correct digit in the correct place, a cow for a correct digit misplaced.
Its best-known descendant is the boxed game Mastermind, which replaced digits with coloured pegs in 1970 and sold in the millions; this catalogue carries that version too, under the name Codebreaker. The word-based variant of the same idea runs ahead of the television game Lingo and of Wordle by decades.
Computers took to it early. A version called MOO was widely available on mainframes and on Multics and Unix systems long before the plastic version reached shops — unsurprising for a game that is really an exercise in extracting the most information from the fewest questions.
Bulls and Cows is a traditional pencil-and-paper code-breaking game in the public domain. This is an original, free browser build with daily and practice modes, adjustable length and a solver-backed hint.