Much of Earth’s energy comes from the Sun. Nearly all life on Earth depends on solar energy since plants use sunlight to make food through the process of photosynthesis. Photosynthesis was the process that fed plants and animals, which in turn, over the course of millions of years, became fossil fuels. The Sun heats some areas of Earth more than other areas, which causes wind. The Sun’s energy also drives the water cycle, which moves water over the surface of the Earth. Some of these types of energy can be harnessed for use by people.

The other main source of energy is Earth’s internal heat. This heat has two origins: the breakdown of chemical elements by radioactivity, and the heat that is left over from when the planet came together. These two sources will be described in more detail in later chapters.

^{Courtesy of NASA. earthobservatory.nasa.gov/IOTD/view.php?id=43717. Public Domain.}