Hey Mr. Green, I usually get 47 to 50 miles per gallon on my 2005 Prius and pay between $2.50 and $2.75 per gallon for gas. Is it cheaper, or better for the environment, to charge gadgets like my cell phone, iPod, and laptop in my car or in my house? I'm in the car about two hours a day. —Joe in San Francisco

I could be lazy and simply tell you to worry less about your toys' relatively small energy use and more about their toxic guts and batteries--and to make sure you safely recycle them. But this is such an intriguing question I couldn't resist poking around for an answer. Though off-the-grid energy production appeals to Americans' do-it-yourself spirit, it turns out that a gasoline engine is not a very efficient device for such efforts.

Using your Prius to charge your cell phone will cause you to emit about 80 percent more carbon dioxide than plugging in at home. (The cost is slightly higher too.) There is, however, one exception: You can get "free" energy if the car's battery is fully charged and the braking system is generating enough power so that the gasoline engine doesn't have to run the alternator to charge the battery. Though this is generally not the case, it might occasionally occur as you roll down the famously twisty Lombard Street or one of your city's other steep hills.

Hey Mr. Green, I am urging my employer to participate in a paper-recycling program. Can you tell me how many trees would be saved by recycling a 30-gallon bin of paper? —Allan in Houston

As teenagers, my buddy Gordo and I whacked scads of innocent trees with our trusty McCullough chainsaw and shipped them to the mill in Dubuque. So toiling to answer this sort of question is a penance for such sins. Better to do it now than to stew in a vat of boiling pulp in the hereafter, taunted by environmental sermons blaring through raspy amplifiers. Anyway, a 30-gallon bin will generally hold around 80 pounds of computer paper, or up to 100 pounds if the paper is tightly packed.

A typical tree used for pulp yields about 83 pounds of office paper, meaning your bin would essentially hold the equivalent of one tree. Since 10 to 25 percent of the mass gets lost in the paper-recycling process, you might not rescue a whole tree each time you fill a bin, but it's safe to say at least three-fourths of a tree could be saved per container. Now if you throw in a lot of crumpled paper that takes up extra space, you'll obviously fall short of that noble goal.

Of course, trees come in various sizes, and some species yield more pulp than others, so these are ballpark figures. Remember too that all paper is not created equal: Virgin office paper requires twice as much pulp per pound as virgin newsprint. But any way you slice it, recycling paper saves a lot of trees.

Hey Mr. Green, Our 800-person office doesn't have a recycling program for beverage containers. We've been told that the empty containers would lure rats and other pests into the building. Do you know how other large companies have solved this problem? —Adam in Indianapolis

The recycling authorities I've contacted have a nice straightforward answer for you: Rinse out your darn bottles, cans, and jars. But even rinsing is not technically necessary. Since the recycling process burns off organic material, dirty containers are mainly a problem when they sit around in hot, humid areas. (If your colleagues are competent enough to keep a tight-fitting lid on the recycling bin, you can get by even in those places without rinsing.)

At the Sierra Club headquarters in cool, foggy San Francisco, that's not an issue, and despite imperfect rinsing habits, we haven't had any pest problems. Not, that is, unless you count the occasional crank caller who informs us we're a bunch of tree-hugging ninnies he'd like to squish under the treads of his ten-ton Hummer.

Of course, pests are not the only issue to consider. Food and beverage remnants can contaminate paper being sorted for recycling in the same facility. (Food waste should be composted anyway.) And think of the hard-working recyclers, sorting your castoffs by hand. As one recycler explains, "We will do our best to recycle a broken glass jar half full of mayonnaise on a hot day, but the spoiled food adds to the challenge."

MORE INFORMATION Read more Mr. Green and submit your own questions at sierraclub.org/mrgreen, or mail them care of Sierra at 85 Second St., 2nd Floor, San Francisco, CA 94105.

Illustration by Melinda Beck