Energy is the ability to do work on matter, according to Michael Ritter, the author of The Physical Environment. Heat, also known as thermal energy, is a type of energy that can be converted from other types of energy. Thermal energy is necessary to sustain life. Natural sources of heat energy can be found in plant and animal products, fossil fuels, the sun and from within the Earth.

The sun is Earth’s major external source of heat energy. The sun’s energy travels to Earth as electromagnetic radiation. The amount of radiation we receive depends on the time of day and season, but it is constantly enough heat energy to support life.

Geothermal energy comes from within the Earth. The heat is produced within Earth’s core, which is made of solid iron surrounded by molten lava. The core is hotter than the surface of the sun. The energy is produced by the radioactive decay of particles of rocks, creating the magma. People use geothermal heat by utilizing hot springs or underground water to heat homes and buildings.

Animal and plant products give us natural heat energy. When we eat hamburgers, an animal source, or salad, a plant source, we get heat energy in the form of calories, which fuels us. When we burn types of plant products, such as trees, heat energy is created. Heat energy from biomass-plant and animal products-is originally from the sun. Plants use heat energy directly from the sun to grow through the process of photosynthesis. Animals eat the plants to get energy. Humans eat plants as well as animals for energy.

Solid fuel, such as coal, and gaseous fuel, such as petroleum, are natural sources of heat energy. These fuels are created over millions of years from the remains of plants and animals. We find them in deposits beneath the surface of the earth. When humans ignite fossil fuels, the fuels combust, creating heat energy.