Fish have developed a neat trick that helps them swim upstream. When water flows past objects in a stream, it develops a string of small whirlpools, or "vortices." A researcher simulated this environment in a tank outfitted with a high-speed camera. They found that fish slalomed in between the vortices and used the energy of each one to help propel them forward — much the same way that a boater uses a sail to tack back and forth in the wind. NPR's Christopher Joyce reports.