You walk into your favourite café with a fixed amount of money in your pocket. The menu has two items you love—say, coffee and pastries. You cannot buy unlimited amounts of both; your wallet draws a line. Meanwhile, your mind has its own map of what makes you happy: maybe you prefer a mix of caffeine and a sweet treat, but after the third pastry the extra joy fades. This chapter explains exactly that everyday balancing act. We build a simple but powerful model that lets us see how consumers choose, why a price cut makes you buy more, and how we can add up everyone’s choices to get the market demand curve managers care about.