[Dr. Scott W. Tinker] This is a rock. But imagine if you could run your electricity system on it. You wouldn't have to put it in a tank like oil or other liquid fuels. You wouldn't have to worry about it floating away like a gas. You wouldn't have to wear a special radiation suit just to handle it. And you wouldn't have to wait for it to come around like sun or wind. It just sits there solid and dependable. Of course, these are exactly the benefits of the rock that burns: coal. It started as plants settle to the bottom of swamps. They were covered by sediment, compacted, and cooked into coal. Throughout geologic time, Earth has been hotter and wetter. Swamps and bogs were widespread, and in all these places today, we find coal. You don't have to refine, purify, or enrich it. You just dig it out of the ground, store it in a heap- for a day or a decade- and burn it when you need it. It's simple, available, and cheap, and that means coal-fired electricity is too. Nearly every Western country developed on it. Now China and India are too. It's the engine that lifts countries out of poverty. Today, coal is the world's leading electricity source, and global use continues to rise. But, for all its simplicity, coal is complicated. That's its molecular structure. There's lots of carbon and hydrogen in there, and when you burn coal, it makes heat. But all this other stuff causes emissions. The nitrogen becomes nitrogen oxides, which cause smog. The sulfur becomes sulfur oxide, which causes acid rain. These together are damaging to human lungs. They are heavy metals and toxins like mercury, lead, uranium, and arsenic mixed in with the coals and enormous volumes of ash that must be disposed of. And, of course, billions of tons of CO2. Coal is our largest source, and in some places, mining coal can be dangerous and environmentally damaging. It's often said there are no benefits without costs- certainly not in energy- and coal was perhaps the clearest example: simple, affordable electricity available in almost every country, offset by the fact that it's dirty. That said, you've probably heard about clean coal, and we'll talk about that next.