Need to charge your phone? Just sing your favorite song. Engineers in South Korea are developing a battery that responds to all kinds of noises. It works by converting sound into electricity. If you’re running out of juice, you could charge your phone by talking on or at it. And yes, you could even belt out your favorite Britney Spears song to extend its battery life. The technology isn’t ready for market yet, but the researchers seem very positive about its potential.