Water harvesting and long term storage Food: In-home organic food production capability The buildings are often horseshoe-shaped due to the difficulty of creating sharp 90 degree angles with rammed tires. In Reynolds's prototype at Taos, the opening of the horseshoe faces 10–15 degrees east of south to maximize natural light and solar-gain during the winter months, with windows on sun-facing walls admitting light and heat. The book, Earthship I, describes how to find the best angle depending on the building's geographic location. The thick and dense walls provide thermal mass that naturally regulates the interior temperature during both cold and hot outside temperatures. The outer walls in the majority of Earthships are made of earth-rammed tires, but any dense material with a potential to store heat, such as concrete, adobe, earth bags, or stone, could in principle be used to create a building similar to an Earthship. The tire walls are staggered like traditional brick work, and often have "concrete half blocks" every other course, to equal the length of the staggered tire below. In an effort to cut down the use of concrete even further, they also use "squishies" - tires rammed in between a tight space to even out the course or to compensate for varying tire size. The rammed earth tires of an Earthship are assembled by teams of two people. One person shovels dirt and places it into the tire one scoop at a time. The other person, who stands on the tire, uses a sledgehammer to pack the dirt in while moving in a circle around the tire to keep the dirt even and to avoid warping the tire. Rammed earth tires can weigh up to , so they are typically filled in place. Because the tire is full of soil, it does not burn when exposed to fire. In colder climates, extra insulation is added on the outside of the tire walls. On top of the tire walls are either "can and concrete bond beams" made of recycled cans joined by concrete, or wooden bond beams with wooden shoes. These are attached to the tire walls using concrete anchors, poured blocks of concrete inside the top tires. Wooden shimming blocks placed on top of the wooden bond beam make up the wooden shoes. The wooden bond beam consists of two layers of lumber bolted on to the concrete anchors. Re-bar is used to "nail"