To build a hot-rock bed, dig a shallow trench slightly longer than the length of your body and about three feet wide. Dig in the driest ground you can find to avoid "steaming" yourself over a damp bed. Line the trench with stones (avoid river rocks, which might be moist inside, because they can explode when heated). Then line the trench with kindling and firewood. Start the blaze and allow it to burn for an hour to heat the rocks thoroughly. After an hour or so, move the remaining fire and coals to your campfire and cover the trench with about three inches of the driest sand or soil available. Allow the heat from the rocks to "cook" out any moisture in the soil covering before lying atop the bed. The warmth from the rocks will rise through the soil and keep you warm as you sleep.