Here's another fun one (from Rosetta Code): play Bulls and Cows! This is basically the game I grew up knowing as "Mastermind." The computer picks a 4-digit number (with no repeated digits), and you have to guess. You score 1 bull for each digit in your guess that is the same place in the computer's number; and you score 1 cow for each digit in your guess that occurs in the computer's number, but in some other position. Repeat until you get it right.
If you pick your guesses carefully, this is enough information to get all four digits in usually a dozen guesses or so. Try it!
