What is energy?

Energy is the ability to do work. Almost all food energy comes originally from sunlight. Every time something gets warm, cools down, moves, grows, makes a sound or changes in any way, it uses energy. In the International System of Units (the SI system), the unit of energy is the joule.
Different forms of Energy can take a number of different forms.
Here are some examples:
Chemical - Chemical energy comes from atoms and molecules and how they interact.
Electrical - Electrical energy is generated by the movement of electrons.
Gravitational - Large objects such as the Earth and the Sun create gravity and gravitational energy.
Heat - Heat energy is also called thermal energy. It comes from molecules of different temperatures interacting.
Light - Light is called radiant energy. The Earth gets a lot of its energy from the light of the Sun.
Motion - Anything that is moving has energy. This is also called kinetic energy.
Nuclear - Huge amounts of nuclear energy can be generated by splitting atoms.
 Potential - Potential energy is energy that is stored. One example of this is a spring that is pressed all the way down. Another example is a book sitting high on a shelf.