The key to cooking meat properly is knowing when to stop. Cookbooks are full of formulas for obtaining a given doneness—so many minutes per pound or per inch thickness—but these are at best rough approximations. There are a number of unpredictable and significant factors that they just can’t take into account. Cooking time is affected by the meat’s starting temperature, the true temperatures of frying pans and ovens, and the number of times the meat is flipped or the oven door opened. The meat’s fat content matters, because fat is less conductive than the muscle fibers: fatty cuts cook more slowly than lean ones. Bones make a difference too. The ceramic-like minerals in bone give it double the heat conductivity of meat, but its frequently honeycombed, hollow structure generally slows its transfer of heat and turns bone into an insulator. This is why meat is often said to be “tender at the bone,” more succulent there because less thoroughly cooked. Finally, cooking time depends on how the meat’s surface is treated. Naked or basted meat evaporates moisture from its surface, which cools the meat and slows cooking, but a layer of fat or a film of oil forms a barrier to such evaporation and can cut cooking times by a fifth.