A simple procedure converts wood to an unnaturally strong material capable of passively cooling buildings, according to a study (Science 2019, DOI: 10.1126/science.aau9101). The finding may lead to “green” construction materials that substantially lower the energy used by buildings, especially for air conditioning. Industry data show that buildings consume 70% of the electricity in the US. Roughly half of it goes to heating and cooling. To address this demand, researchers have spent years developing materials that dissipate heat without consuming electricity. Some of those materials mediate passive cooling fairly well at night, by emitting thermal energy as infrared light. But daytime cooling remains challenging because those materials tend to absorb more energy, in the form of visible light, than they can dissipate. Xiaobo Yin of the University of Colorado Boulder, Liangbing Hu of the University of Maryland, College Park, and coworkers have taken steps toward a solution. Using hot hydrogen peroxide, the team delignified wood, then pressed it, forming a material with densely packed, partially aligned cellulose nanofibers. The bright-white product strongly reflects solar light instead of absorbing it, while dissipating heat by emitting mid-infrared wavelengths. Modeling studies show that the densified wood, which is more than eight times as strong as natural wood, could reduce energy costs by 20–60%, depending on the application.