Turning on the air conditioning to its highest setting can bring sweet relief. But your resulting energy bill? Not so much. What if your home could stay cool all on its own—no electricity required? That’s the premise of Yi Zheng’s new invention. The associate professor of mechanical and industrial engineering at Northeastern University in Boston has created a sustainable material that can be used to make buildings or other objects able to keep cool without relying on conventional cooling systems. Zheng envisions this material, dubbed “cooling paper,” covering the roofs of houses, warehouses, and office buildings. Not only does the light-colored material reflect hot solar rays away from the building, it also sucks heat out of the interior, too — heat that is emitted from electronics, cooking, and human bodies. Cooling paper is, in fact, made of paper, and the porous microstructure of the natural fibers inside absorbs the indoor warmth and re-emits it away from the building. Zheng, who studies nanomaterials, got the idea when he looked at a bucket full of used printing paper.