that result are often strategic, dependent upon changes in climate, other competitors, who happens to be in a bad mood, and who is sexually active. In a variety of species, from dung beetles to deer, biologists have developed mathematical models that accurately predict how long an individual should wait for a sexual partner or feed in a food patch. The accuracy of these models shows that individuals’ desires for resources linked to survival and reproduction are captured by lawful principles or rules. This is important because it means we understand how the machine underlying behavior works. It means we understand how animals make certain choices. Understanding how wanting works is straightforward. In both humans and nonhuman animals, we can measure what individuals approach when we give them a choice, as well as how much effort they are willing to exert while approaching and gaining access to a particular object or experience. For example, in studies that explore whether captive animals are provided with sufficient housing conditions, an experimenter presents individuals with a choice of rooms, one consisting of the typical housing environment and the others by the addition of goods believed to be of interest. To enter a given room requires opening a door. To determine how much an individual really wants what is in another room, the experimenter ramped up the difficulty of opening each door. In studies of captive hens and mongoose, individuals exerted considerable effort to open some doors but not others. Hens rammed into doors opening onto a chipped wood floor, whereas mongoose did the same for a pool of water. These are items they want, but do not get in captivity. What about liking? It may seem, at first blush, that because liking is a subjective experience, that there are no clear objective ways to measure it. My likes are my own. You can’t possibly know what it is like to be me. If you can’t know what it is like to be me, then we can’t possibly know what it i