Tired of having to remember to charge your cell phone every night, or of running out of juice in the middle of a call? Scientists may have come up with a solution for you: just go for a walk.

Oddly enough, I’ve never seen this kind of thing mentioned in science fiction. It seems obvious in retrospect. But then, most science fiction assumes that perfect batteries with enormous capacities will be developed, and plugging a phone into a wall socket or the lighter socket of a car every couple weeks won’t be a major hassle.