Bulls and Cows

A simple yet fun code-breaking game with origins dating back to at least 19th century. It is also the predecessor of the famous board game Mastermind.
Bulls and Cows is a paper and pencil game for two players. On a sheet of paper each player writes a 4-digit secret number where all digits are different. Preferably first digit is non-zero number. (If you are curious to know, there are a total of 4536 numbers satisfying these conditions, the smallest being 1023 and the biggest 9876). Players take turns to make guesses and receive clues from the opponent in order to find out the secret number of opponent. When a guess made, the player compares the guess to his/her secret number to see how many digits of the guess matches the secret number. The matching digits in their right positions are "bulls", and those in different positions are "cows". For example when the secret number is 2483 and the guess is 8253, there are only one BULL (3) and two COWS (2 and 8). So the clue in response to the guessing player is ONE BULL and TWO COWS. If none of the digits match, the holder of the secret number says: NULL. The player to first find out opponent’s secret number wins.
In Bulls and Cows, accuracy in giving the clues is as important as making good guesses. So it is recommended to keep a neat record of guesses and clues by both players (you can download a Bulls and Cows template sheet below). If it turns out, at the end of the game, that a player gave an inaccurate clue at some point, then that player is deemed defeated de jure.
If you are rather cautious about burning a brain with numbers, there is a word variant of Bulls and Cows as well. Players have secret words with certain number of letters, instead of numbers of a given number of digit-length. In the word play, all letters of a secret word should be different and guesses should be meaningful words rather than a random combination of letters.