You're halfway through listening to "Layla" when it happens: Your MP3 player's battery dies. Normally you'd have to wait until you were at your computer to finish rocking out, but there is an easy and eco-friendly way to do it on the go. First, slip a piezoelectric transducer -- a device that generates a tiny charge when touched -- into your shoe. A connected module collects the voltage created every time you take a step and continuously powers up a rechargeable AA battery. (It takes a lot of walking to get a full charge, but it's perfect for reviving or topping off a gadget.) Once the battery is charged, put it into a DIY five-volt converter, and plug in your dead MP3 player. Now you can listen to the guitar solo while you walk some more juice into another battery.