Sod houses are an almost vanished part of America's past. Yet in Nebraska and some other states a century ago they were very common. Sod houses were built, as the name suggests, from sod - the top, grassy layer of the ground. The ground of the Nebraska plains was particularly suitable for this, because it was held together by the thick wiry roots of the grass that grows there.To make a house, the settlers used a sod-cutter, drawn by horses, that cut through the top layer of the grassland. This had the additional benefit that the land cleared of grass in this way was suitable for planting corn. The builders would then cut the top layer they had ploughed into sections of about 50kg each, clearing about two acres of grass in the process. These sections were stacked on top of each other to make a thick-walled house that was cool in summer and warm in winter. Because many were built into the side of hills for extra stability, they were even useful shelters from tornadoes.