Fossil fuels are stored solar energy. Plants grew millions of years ago, and then were buried and slowly converted to oil, natural gas or coal. The uranium that powers nuclear power plants ultimately came from supernova explosions billions of years ago.

There are two basic reasons why it matters whether you use renewable energy or fossil fuels. First, there is a limited amount of fossil fuels available and we are using them up much faster than Mother Nature is making them. It took many millions of years to make the petroleum  we use to run our transportation systems-- and in less than 200 years, we have used up half of the reserves on the planet. Some forms of fossil fuels are almost already gone. Second, when we use fossil fuels we burn them and burning fuels causes air pollution. Some air pollution causes smog, some toxic effects, and some cause global climate change. We are running out of clean air even faster than we are running out of fossil fuels!