A resource is something that humans consider valuable in their lives and hence has a purpose. Many resources, such as water and energy, are classified as natural resources. Natural resources are divided into two categories: renewable and non-renewable. Thus, resource geography can be described as the study of resource distribution and features that distinguish one place from another, with a focus on resource consumption, evaluation, conservation, and management in relation to the environment. It includes a study of man's prospecting for natural resources, as well as their use and development. Geo-resources are used in almost every aspect of our everyday lives; it's not simply the gasoline that powers our vehicles and buses, or the natural gas that heats our homes. Geo-resources are also required for the development of long-term energy sources. Solar cells, wind turbines, electric motors, and, last but not least, our mobile phones all use rare earth metals. Geothermal energy is the thermal energy in the Earth's crust that comes from both the planet's formation and the radioactive decay of elements in currently unknown but possibly nearly equal quantities. Some rock melts and the solid mantle behaves plastically due to the extreme temperature and pressure in the Earth's interior. As a result, because the mantle is lighter than the surrounding rock, it convects upward. The core-mantle interface can reach temperatures of around 4000 °C (7200 °F). Geothermal power, the term for the generation of electricity from geothermal energy, has recently increased in popularity. The earth's geothermal resources are projected to be more than sufficient to meet humanity's energy demands, yet only a small fraction is now profitably explored, often in locations near tectonic plate borders.