Coal is a special kind of rock that can burn. It comes in colors like black or brownish-black, and it's made from plants that lived a really, really long time ago. 

We call it a nonrenewable energy source because it takes millions and millions of years to form. 

Inside coal, there's a lot of energy that comes from plants that lived in swampy forests hundreds of millions of years ago. 

When coal burns, its carbon combines with oxygen in the air and forms carbon dioxide. 

Carbon dioxide (or CO2) is a colourless, odourless gas, but in the atmosphere, it is one of several gases that can trap the earth's heat.

This is know as the greenhouse effect.