Xiuhtezcatl Tonatiuh became a climate change activist at age 6 when he saw an environmental documentary. He asked his mom to find a way for him to speak at a rally. Now 15, the long-haired, hip-hop-savvy Coloradan is one of 21 young activists joining climate scientist James Hansen in suing the Obama administration for failing to ditch fossil fuels. "It's basically a bunch of kids saying you're not doing your job," he told me here at the U.N. COP21 climate change summit in Paris." You're failing, you know. F-minus.