Energy

The important thing to know about energy is that it takes many forms, but it never simply appears or disappears. If a system (you, or a tennis ball, or a tree) gains energy, that energy came from somewhere, and if it loses energy, that energy has gone somewhere; in both cases, the energy often changes form as it moves from one system into another. In fact, energy often changes form even while it stays within one system.

Anything that is moving has energy; more energy means more motion. Sometimes that motion is happening on a microscopic or sub-microscopic scale, as is the case with thermal energy. Another important form of energy is potential energy - think of a stretched rubber band or a compressed spring, or a bucket of water balanced above a door.

If you see something, or hear it, then energy moved from it to you, as light or sound, respectively. If something changes shape, then its energy changes; this means it gives off energy or absorbs it.

There are some particularly important forms of potential energy, such as chemical potential energy (in coal, or food, or batteries), or spring potential energy (a situation where the deformation energy is elastic), or gravitational potential energy. There is also electric potential energy; if you've reached for a doorknob in wintertime and created a spark, then you've witnessed a conversion of electric potential energy.