The most important mechanism of climate change can be understood by everyone: Why do greenhouse gasses have such a direct warming effect on our planet? This chapter approaches this question with a Do-It-Yourself (DIY) attitude. First, the humorous tale of Stinky, Dinxie, Bif, and Moo teaches us how the greenhouse effect really works. It's a straightforward matter of balancing energy, not a matter for belief. Also, it turns out that the atmosphere is really thin, and has a lot less actual mass than we might at first think. Then, this understanding is augmented by lots and lots of data. Multiple independent data sources hammer home convergent evidence identifying very rapid levels of observed warming. Looking at air temperatures, ocean temperatures, and global sea levels, we see extremely rapid rates of warming, rates that have increased dramatically in the last decade. 2015–2019 stand out as exceptionally warm. Global temperatures are modeled extremely well by climate models, while the observed warming doesn’t track at all with changes in incoming solar radiation, and these changes are very small energetically. We don’t need to believe in climate change; we can understand and observe it. The chapter introduction and a sidebar use the devastating Thomas Fire to set this warming in context.