Bulls and cows: crack a secret number using only 'hot' and 'close' clues
One player picks a secret number with no repeated digits, four digits is the classic length. The other player guesses, and after each guess is told only two counts: how many digits are correct and in the right position ('bulls'), and how many are correct but in the wrong position ('cows'). No other information is given, not which digits, just the two counts. Using nothing but a sequence of bulls-and-cows scores, how do you deduce the exact secret number?
Reveal the answer
By guessing systematically and treating every prior guess's bulls-and-cows score as a constraint the hidden number must satisfy, a skilled player can always solve a standard four-digit version in well under ten guesses. This old pen-and-paper game, of unclear exact origin, became the direct ancestor of the commercial board game Mastermind, invented by Mordecai Meirowitz around 1970 and released by Invicta Plastics soon after, which simply swapped digits for colored pegs.