One of our readers wrote in with this question: I know that eventually oil will run out and that we'll need to develop alternative sources of energy. And I keep hearing about things like wind and solar power and hydrogen fuel cells. But which of these seems most likely to emerge as a major source of energy in the future?

Here's the answer:

Wind and solar power will most likely play roles, as well as nuclear power. But there's another potential source of renewable energy that you don't hear as much about-forests.

Some energy scientists today see forests as sources of renewable energy. For example, forest biomass, meaning leaves and stems and small trees and other plants, can be used to make ethanol. Forest biomass can also be used to produce charcoal-like pellets that can be burned for energy. The best part is, we won't be chopping down all trees at once.

Forest scientists know that cutting out smaller trees and plants is actually good for forests. When forests become too crowded with trees competing for light and water, biodiversity tends to suffer. So careful thinning out of forests can be healthy.

Maybe one day we'll be running on forest power.