In the age of mp3s, most people have a lot of old CDs lying around. Instead of throwing them in the trash, though, you can put them to use in a way that's both eco-friendly and healthy. Thread the discs onto the ends of a thin metal rod and secure them in place with bolts to make a usable (and cool-looking) dumbbell. Tape up the middle of the rod to provide a comfortable grip, and you'll be all set for a workout.
Five amazing, clean technologies that will set us free, in this month's energy-focused issue. Also: how to build a better bomb detector, the robotic toys that are raising your children, a human catapult, the world's smallest arcade, and much more.