The food that plants make is the food on which all the animals in the world depend in order to live. Animals and human beings use the energy found in food to operate their bodies and muscles. The sun's energy is stored up in coal, wood, and oil, which people burn to do work for them. The sun evaporates the water which falls as rain. This causes rivers to flow and produces other energy that people can use.
Energy is one of the two fundamental ideas in physics.
The other is matter. These two ideas are not completely separate. Many physicists believe that energy and matter are merely two aspects of the same thing, much as ice, water, and water vapor are three different aspects of water. A body loses part of its mass when it releases energy. It gains mass when it absorbs energy.