Energy is the ability to do work. Scientists define "work" as when a force moves something. When you lift a brick, you are doing work because you are exerting a force to lift the heavy object. Energy comes in many forms, such as heat, sound, light, electric, and chemical. Energy easily changes from one form to another.

What it is: the ability to do work
Main kinds of energy: potential and kinetic
Main units of measurement: joules or calories
Different forms: atomic, chemical, electric, mechanical, sound, heat, and light
Characteristics: changes from one form to another; can never be destroyed

Image credits: main image, © AMNH/Eric Hamilton.