900. What's energy?
Formally defined as "the capacity to do work", energy is a measure of an object's ability to make things happen. It is interesting to physicists for one important reason: it is a conserved physical quantity. By "conserved physical quantity", I don't mean that it's something that we try not to waste. I mean that the amount of energy in an isolated system can't change—energy can't be created or destroyed, it can only be transferred from one object to another or converted from one form to another. Because you can't make it or consume it, energy is an important characteristic of objects and systems. You can often watch it move from object to object and observe the consequences of this movement. For example, the energy that I'm using now to type at my keyboard arrived at the earth's surface as sunlight, was used by plants to build new molecules that eventually become part of my breakfast this morning and are now being combined with oxygen in my body to allow me to move my fingers. Nowhere along this chain was energy created or destroyed—it simply moved about and changed forms. It will still be here tomorrow, and then next day, and even the day after that.