Among recent high profile scientific debates was the proposal that life could exist with arsenic in place of phosphorous in its nucleic acids and other biomolecules. Soon after its initial publication, scientists across diverse disciplines began to question this extraordinary claim. Using the original article, its claims, its scientific support, and the ensuing counterarguments, a two-day, active learning classroom exercise was developed focusing on the presentation, evaluation, and discussion of scientific argumentation and discourse. In this culminating assignment of a first semester biochemistry course, undergraduate students analyze the scientific support from the original research articles and then present and discuss multiple scientific rebuttals in a lively, civil classroom debate. Through this assignment, students develop a sense of skepticism, especially for the original arsenic-based life claims, and learn to clearly articulate their counterarguments with scientific support and critical reasoning. With its direct integration into first-semester biochemistry curriculum and the excitement surrounding arsenic based life, this assignment provides a robust, simple, and stimulating framework for introducing scientific discourse and active learning into the undergraduate molecular science curriculum. (c) 2016 by The International Union of Biochemistry and Molecular Biology, 45(1):40-45, 2017.