This toy car contains a fuel cell and runs on hydrogen gas, one of the contenders for the fuel of the future. It also demonstrates the conundrum of building a "hydrogen economy": The gas is a fuel but not an energy source. You have to make it first. The Hydrocar's fuel cell splits water into hydrogen and oxygen to create its own fuel. To power that reaction, the kit comes with a solar panel, but you need direct sunlight to make it work.

The fallback is regular alkaline batteries. Kit requires some assembly and is for experimenters ages 12 and up, under adult supervision. The risk of Hindenburg-type flameups appears small. Making hydrogen is fun, but it's not something you'll do again and again. ($85 from Horizonfuelcell.com)

• Dahon Curve D3

Bicycling is a great way to get around, but parking and storage can be difficult, particularly in cities. Folding bikes are one solution, and they aren't as weird as they sound. The Curve D3 folds up into a tight package, yet feels sturdy and comes with city-bike conveniences like wheel guards and a rack. It has three speeds, enough for city riding (unless that city is San Francisco). At 27 pounds, it's not something you'll carry around easily, but you'll have to pay a lot more for a lighter bike. It's a big seller at NYCeWheels.com/A>, a New York store that specializes in folding and electric bikes. ($399.95)

• Solio Hybrid 1000

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This solar charger for cell phones and MP3 players isn't a really environmental gift, since it doesn't save much power. Let's just say it's inspired by environmentally friendly technology. It's about the size of a TV remote, and could be useful for hikers or those with an off-grid cabin, but be aware that it takes a long time to charge up the internal battery using sunlight.

The manufacturer says it takes 10-12 hours of direct sunlight (with no intervening glass) to charge it up. The internal battery can then fully charge a cell phone. If you have no sunlight, you can charge the internal battery via a PC's USB port. Any phone that can be charged via USB, plus Nokia phones, can connect to the Solio. We were able to charge a BlackBerry but not a Sony Ericsson phone. Poor winter weather precluded us from testing the solar panel. ($79.95 from www.solio.com)

• Thames & Kosmos Power House

This amazing kit lets you build a prototype home of one square foot and test several technologies for sustainable living, including solar heat, greenhouse gardening, air conditioning by evaporation, electric cars and desalination of sea water. You can boil an egg with the included solar cooker. A 96-page manual details 70 experiments. Since it's a translation from German, you know one of them is turning cabbage into sauerkraut.

Oddly, the house is made of foam plastic, which isn't biodegradable, but at least its not meant to be immediately disposed of, like plastic packaging. Good gift for budding architects and anyone ready to move up from Lego, age 12 and up. ($140 and up at online stores.)