Energy is the power used to do work or to produce heat or light. Energy cannot be created or destroyed but it can be changed from one form to another. For example, when a piece of coal burns, the energy locked up in the chemicals in the coal is converted to heat and light.
Energy comes from the sun and is called solar energy.
Living plants are able to convert solar energy to chemical energy by a process called photosynthesis. This chemical energy is used to make other substances such as protein, carbohydrate and fat, all of which provide energy. Animals cannot use solar energy directly but can use the chemical energy contained in plants or other animals. When energy is being used some heat is nearly always generated. Energy can be measured in either joules or calories.