At moderate and hot oven temperatures, the oven walls, ceiling, and floor radiate heat energy in significant amounts. This means that if an object lies between the food and one of the oven surfaces, the food will receive less heat from that direction and will cook more slowly. This shielding effect can be both a nuisance and a useful tool. The pan underneath a roast slows the heating of the roast bottom, and the cook should turn the roast periodically to make sure that top and bottom receive equal amounts of heat. But a sheet of aluminum foil deliberately placed over the meat will deflect a substantial portion of heat energy and thus slow the cooking of the whole roast. So will basting with a water-containing liquid, which cools the meat surface as it evaporates.