Rice, the main course for most meals for half the world's population, can be grown in paddies or on dry land. Dry land cultivation uses a "slash-and-burn" method of preparing land for planting, a method which requires yearly location changes -- to find new fields to slash and burn and plant with rice. This method only yields enough rice to support between 13 and 52 people per square kilometer (35 to 130 per square mile). Paddy rice cultivation, which was perfected in South and Southeast Asia, requires constant attention, year-in and year-out, but creates a dependable, plentiful food source for an enormous population. Feeding large numbers of people on one crop, however, requires that large tracts of land be given over to that task. So, the choice became wood or rice, and rice won. This makes for lots of food, but not a lot of fuel. Enter the southern Chinese invention of wok cooking. A wok reaches a very high temperature rapidly, and distributes the heat evenly,which allows for quick cooking of virtually anything. The faster you cook vegetables, fish or meat, the more fuel you have left to cook large quantities of rice.