Alternative fuel sources are all the talk these days. But what exactly does that mean? At the National Arboretum in Washington, D.C., an exhibit of "Power Plants" showcases 21 plants that have the potential to become biofuel.

The exhibit sets out to answer: Which plants would make the best biofuel? How much irrigation does the plant require? How much space does it need? Is the plant already in demand as a food source?

The 21 plants each have traits that could make them viable as biofuel. Barley and corn are high in starch. Sunflowers and soybeans are high in oil. The sucrose in sugar cane is a relatively simple and cost-effective form of biofuel. Even algae, high in both starch and oil, have biofuel potential. (Mustard seeds are 25 to 40 percent oil and can be used to create a low-emission fuel. But mustard is a cold-weather plant, so you won't see it growing at the arboretum at this time of year.)

The outdoor display may leave you with a few unanswered questions: How DO oil, starch and sugar become fuel? Or, how many of these plants are scientists seriously considering as biofuel? (How could this work in Hawai'i?) But the exhibit is food for thought. Plus, you're outside, and exercising both your body and mind.

The National Arboretum grounds are open daily (except Christmas) from 8 a.m. to 5 p.m. The exhibit runs until Oct. 15.

3501 New York Ave. NE, Washington. Free.