In which we discuss refrigeration, how it works (more or less), and the history of refrigerants, with their various advantages and disadvantages for the environment. Ammonia is toxic. Propane is flammable. Freons and hydrochlorofluorocarbons (HCFCs) destroy the ozone layer and contribute to global warming. Hydrofluorocarbons (HFCs) are also greenhouse gases, except for (HFO)-1234yf, which has a global warming potential (GWP) lower than carbon dioxide, but it is flammable and produces highly toxic and corrosive hydrofluoric acid (HF) or toxic trifluoroacetic acid. Finally, the greenest alternative is carbon dioxide itself, non-toxic, non-flammable, non-ozone depleting, and a relatively low GWP. The supercritical form of carbon dioxide is discussed, as well as its engineering advantages and disadvantages as a refrigerant. Hosted by Felicia Etzkorn (Virginia Tech) and Jamie Ferguson  (Emory & Henry College), with music by Wendy Godley of The Kind.