Energy is the ability to do work. A Joule, the metric unit of energy is a kilogram- meter- square- per- second- square. Both of those ideas can be difficult to wrap your mind around. The easier way to think of energy is perhaps by its various types. You should have an intuitive feeling that a fifty pound rock held above your head has more energy of position in a gravitational field than the same fifty pound rock by your feet. A rubber band pulled back has more spring energy than a lax one. A speeding train has more energy of movement than a still one. We usually value petroleum not for its beauty, but for its chemical energy content. Energy is transferable from one type to another, but is not lost or gained in changes.