Chinese cooks have long used pans and utensils, such as the wok, where the shape and function have remained almost unchanged for centuries. The natural materials of clay, metal and bamboo are still preferred for the preparation and serving of food, and the most popular cooking methods, such as stir-frying or steaming, rely on direct heat; convection ovens are not often used in Chinese kitchen. The few modern culinary inventions found in Chinese kitchens, such as electric rice cookers, tend to be updated versions of traditional equipment and techniques.