Game design is about crafting a micro-economy. Even in an action game, every action the player or an enemy can perform is a stock or a bond or a unit of currency that can be traded for something else. As the screen scrolls from left to right, Super Mario Bros.’s market fluctuates. One fireball can buy one dead koopa. The points at the top of the screen can’t buy anything. Given enough time and a spreadsheet, I could tell you how many dead Goombas a Bowser fireball is worth.