That little extra spring in your step might turn out to be useful. According to a post on Science Magazine’s Science Now blog, Tom Krupenkin and J. Ashley Taylor — two engineers at the University of Wisconsin, Madison — have developed a plan for a device that will convert mechanical motion (walking or running, for instance) into enough electricity to put a charge in your cell phone. It’s sort of a super-advanced version of those light-up hightops that were popular back in the ’90s. Of course, the technology (which involves pressing droplets of liquid metal against an electrode to harvest electricity) is a little more complicated than that. But, according to Krupenkin, if it was scaled to a size that would fit into a typical shoe, you could recharge a normal cell-phone battery with a two-hour stroll. Of course, you’d have to plug your phone into your shoe.