To build an imu, a lua or round pit, about 2 feet to 4 feet deep with sloping sides, is dug into the earth. The diameter and depth of the lua will match the amount of food to be cooked. The pit must be large enough to contain not only the food, but the rocks and the vegetation. Keep the imu as compact as possible. Place the excavated dirt next to the pit. Later, it will be used to cover the imu. Next, gather kindling material, like twigs, small branches, and any other combustible tinder. Place the kindling material in the bottom center of the pit. Larger wood (preferably hardwood) is built around the kindling wood. Do not use wood that will impart an unpleasant taste to the food. Stones, about the size of a closed fist, are then positioned on top of the larger wood. Vesicular basalt stones are ideal for imu cooking. These porous rocks retain heat and are less likely to break. Stones that contain moisture, which causes the rock to explode when heated, should be avoided. The kindling wood is lighted and the blazing fire heats the pit and the stones. As the wood turns to charcoal, the imu stones drop inward on the hot coals. Firing time varies from 1 1/2 to 3 hours until the stones are at their maximum heat. The hot stones are then leveled out with a stick or wooden tongs to an even floor on top of the coals.