We're finally back, and this time we're talking everything thermal! After some brief updates, we dive into blackbody radiation, Planck's law, and selective emitters. Then Jay explains how to use these concepts to design incandescent lightbulbs that are actually twice as efficient as LED bulbs. Then we move on to one of Jay's all-time favorite ideas: passive daytime radiative cooling. And things get even crazier when Jay explains how to use the universe as a really great cold reservoir in a heat engine to get harness solar energy without any photovoltaics.