One of the oldest methods of sustainable building is to use the earth in the construction of the walls of a home. Above is an example of putting soil into sacks that are used to line the walls of a home or building. Doing so increases the thermal mass of the walls of the building. This helps to naturally keep the building cool in the summer and warm in the winter. Another type of earth home is a home built with adobe construction, since the adobe bricks are simply baked clay earth. The same result can be obtained by sinking the majority of the building into the ground or by constructing the building with an earth roof.