In "Cute Poison", Michael hid his chemical substances separately in two toothpaste tubes. Later, he poured both of the substances into a pipe and the mixtures reacted violently and corroded the metal. Where was the pipe? At the ___.

Michael uses the chemical reaction in the infirmary's pipe or drainage system as part of his escape plan. By corroding the metal with the violently reacting mixture, he creates a structural weakness that serves his larger purpose of eventual escape. The infirmary setting is strategically chosen because Michael needs regular access there for his fake insulin shots, giving him repeated opportunities to work on his plan.