Surveys have shown that a proportion of the American public accepts pseudoscientific claims as scientific facts. The critical evaluation of these claims via classroom discussion about pseudoscience is important because instructors and students cannot test every pseudoscientific claim. Instructors can provide a framework to use in evaluating such claims both inside and outside the classroom, arming students with knowledge. I describe an activity that provides both an example of a pseudoscientific claim — astrology — and a framework for classroom discussion of pseudoscience as students work in groups to experimentally test predictions of horoscopes. The activity is appropriate for freshman college students in an online or classroom biology laboratory and can be adapted for high school students.