Read the passage. Earthships ­­­­­What is an earthship? An earthshi­­­p is a home designed to make use of recycled materials and increase energy conservation. Ideally, by living in an earthship, a person can have a home that is "off the grid. " People who live in earthships do not depend on outside sources for electricity, food, or water. Building an earthship may be an attractive choice for those who want to use fewer of our planet's non-renewable resources. The Foundation First, the foundation is built by firmly packing dirt inside recycled tires. Then, the tires are placed in a pyramid-like stack going as high as needed. Next, cement is spread and smoothed between the tires to create a solid wall. When this is finished, the walls are sealed with a protective coating and painted. Some interior walls are built using recycled cans and cement, also sealed with a protective coating. Solar Energy Solar panels give enough stored energy in batteries to provide electricity for appliances, lighting, and electronics. However, solar-powered batteries hold about one-third the charge of energy used in a regular household wired for electricity. This means that people either buy more batteries or intend to use less electricity than a conventional household. Ideally, earthships are built in places where there is an abundance of sunshine year-round. Floor-to-ceiling windows on an earthship's south wall also allow plenty of sunlight. It is important that no trees block the light on this side of the house. The sunlight shines directly onto a brick floor, which then absorbs the heat. This provides enough warmth to maintain a comfortable temperature in the house for the rest of the day. This method of heating the home is called passive solar. Most earthships also have either a wood stove or a heater that uses propane, a type of natural gas. These heat sources are useful on cloudy days. During the summer, the combination of the cool temperature of the earth beneath the floor plus the thick walls can keep the house comfortable.