This is one of 30 lessons from the NSTA Press book Scientific Argumentation in Biology. The lesson engages students in an argumentation cycle by posing a question on the influence of the environment (light) on the genotype or phenotype of the tobacco plant. The lesson provides three possible answers to the question and students design and conduct controlled experiments using tobacco seeds to determine the most valid explanation. Students are challenged to collect evidence needed to construct an argument defending their claim of environmental influence on tobacco plants. Students need prior knowledge of Mendelian genetics to construct a valid argument. Students develop a whiteboard presentation to justify their claims and share their work using a round-robin format. Teachers are encouraged to refer to the preface, introduction, assessment samples, and appendix provided in the full book for important background on the practice of argumentation and resources for classroom implementation. The standards addressed in the lesson are also included in the teacher’s notes.