Though our electricity system is highly complex, its basic principle is simple: 99 percent of our electricity comes from turning a generator.

We do that mostly by burning a resource like coal or natural gas to boil water, which makes steam, which turns a turbine, connected to a generator.

Heat from a nuclear reaction or a geothermal well are other ways to make steam and turn a generator.

Water held behind a dam, then released to flow through turbines, turns generators without having to produce steam.

All these generation systems produce emissions—like water vapor, CO2 or other gases, particulates, or a small amount of nuclear waste.

And all of them are available on demand, which is very important, because we can’t store electricity very well at scale. So it must be made when we need it.

Wind, too, turns a generator. It makes up about 1 percent of global power generation.

Solar, the only one to produce electricity without a generator, makes up another 1 percent.

Wind and solar produce no emissions. But they have other environmental impacts, in mining materials, manufacturing, the large amounts of land they occupy, and eventual disposal.

And because they make electricity only when the sun shines or the wind blows, we have to back them up with other power sources.

The modern world depends on our electricity system, and it’s something we’ll talk more about.