When people interact with one another, they usually behave in a specific, precisely describable way: provided that they act rationally, they each play a game according to certain rules. This occurs whenever strategies are applied and when there is something at stake to win. This applies to economics as well as to evolutionary biology, online poker, behavior in stock markets, romantic relationships, or political negotiations. This is where game theory comes into play, helping to better understand the underlying strategies: Why and when do we choose something? Why do we play fair at certain times? When do we give up?.