Heat also travels from the outside to the center of a solid piece of food—a piece of meat or fish or vegetable—by means of conduction. Because the cellular structure of foods impedes the movement of heat energy, foods behave more like insulators than like metals, and heat up relatively slowly. One of the keys to good cooking is knowing how to heat a food to the desired doneness at its center without overheating its outer regions. This is not a simple task, because different kinds of foods heat through at different rates. One of the most important variables is the thickness of the food. Though common sense might suggest that a piece of meat one inch thick would take twice as long to cook through as a half-inch piece, it turns out that it takes somewhere between twice and four times longer, depending on the overall shape: less for a compact chop or chunk, more for a broad steak or fillet. There’s no absolutely reliable way to predict how long it will take heat to move from the food surface to its center, so the best rule is to check the doneness frequently.