It's a very easy game that isn't particularly easy on humans - but, as @mniip has demonstrated, is easy for computers. Describing it here because I need node-irc-bot to describe it to people but it's not good at long articles.
Requirements:
- Paper, pen / text editor (you'd probably want to take notes)
- At least two participants
The first player thinks of a four-digit number with no repeating digits. (For example, 1234).
The second player must now guess that number.
The second player tells the first player a guess (For example, 4932).
The first player must then respond with:
- The amount of numbers that the two numbers share (eg 4932 and 1234 both contain 2 and 3)
- The amount of numbers that share a position (eg there's a 3 in both numbers at position 3)
For example, the response for guessing 4932 will be 3, 1, while the response for guessing 5674 will be 1, 1.
The gameplay repeats from this point.
The winner is the player who can find out their opponent's number in the least amount of guesses.
I usually play it with friends in that everyone makes up a number and everyone guesses the number of the person to their right. This allows for potentially infinite players, while not making it too easy by allowing anyone to guess one player's number.