The most common question astronauts get is how they go to the bathroom in space, and the answer starts with physiology: in weightlessness fluids shift toward the chest and head, the body reads it as overload, and within about two hours of launch the kidneys demand relief. This episode explains why the space toilet is priority one and how engineers replaced gravity with airflow, a kind of reverse air hockey suction, including the vacuum hose with anatomy specific funnels, the slits that keep a flush fitting funnel from forming a painful seal, and the gas permeable bags that let air through but trap solids before waste is freeze dried by exposure to vacuum.

Then we trace the uncomfortable history. Gemini and Apollo crews used relief tubes and taped plastic bags to themselves, kneading in bactericide to stop methane from bursting the bag, and  ...