Energy is really the power to make something happen. When scientists define it as 'the capacity to do work', they mean the same thing. Energy comes in many forms, from the chemical energy locked in sugar to the mechanical energy in a moving bicycle. But it always works in two ways - energy transfer and energy conversion. Energy transfer simply means it moves from one place to another, as it does when you throw a ball or smoke rises.
Energy conversion means changing from one form of energy to another.
Stored and moving energy Energy is sometimes divided into two kinds. Potential energy is energy stored up ready for action. There is potential energy in a wound-up rubber band, in a squeezed spring and in food, coal, wood, oil and other fuels. There is potential energy too in things far above the ground, because gravity could make them fall. A rolling ball has kinetic energy.