 Access to available energy is absolutely critical to our modern life. It is a dominant force in international and domestic politics, and you might ask the question, if energy is conserved, how can that possibly be? And the answer is fairly simple. Energy has a lot of different forms, and some of those are really easy to use, and some of those are basically impossible to use. So for the vast majority of human history, the main energy source that we could access was the energy stored in our own muscles. Or if we're going to get the biology right, more specifically I guess the bulk of it was stored in the liver, in the form of glycogen, or in fat cells in the form of fat. Now how did that energy get there? Well that's a long story, essentially it came from nuclear reactions in the sun into sunlight, light down in through the sky into plants, plants converted into sugars, the sugars were either eaten by animals or by us, and then the animals were eaten by us, and somehow all that got together into our glycogen and fat stores, and we could run, jump and adjust our environment to our heart's content. And even of course once we use that energy, the energy isn't gone, it's just changed form. So what's happened is that chemical potential energy in the chemicals in our body has turned into some kind of useful work, and some waste heat. It was actually the unbelievably important discovery that heat was a form of energy that allowed us as a species to go beyond the limits of what could be done with just the chemical energy stored in our own muscles and the muscles of any animals that we domesticated. Besides perhaps the occasional water wheel, all energy that we were using was coming from us, and this is a huge limitation even if we were using machines to try and use that energy and do that work as efficiently as possible. So instead when we discovered that heat was a form of energy, we learned not only that we could put work and create heat, so we could say bore out a cannon and make it hot in the process, we also discovered how to do the reverse, which is how to use a fire to make say a steam engine and create work and useful energy out of making things hot. When we discovered we could burn things in some way and make them hot and then use that to do work, that's what changed the world. That's how we got from millions of years of very stable technology to suddenly the industrial revolution and things changing on an enormous scale.