Some games have a magical quality that makes you want to play them time and again. Game designers call it playability. To make a game with great playability, you need to think about all the ingredients that make up the game and how they work together. Games don't have to be complicated to make people want to play them over and over again. One of the first successful computer games was a simple tennis simulator called Pong. The ball was a white square and the rackets were white lines that could only move up and down. Although there were no fancy graphics, people loved Pong. They could compete against friends, just like real tennis, and it was just hard enough to demand intense concentration and a steady hand, leaving players always wanting another game.