Short metacognitive prompts--like "minute papers"--are simple enough to be widely adopted by instructors. But do they work? We investigate how they affect college students' performance in quantitative (Physics) and qualitative (Psychology) courses, comparing classes which received metacognitive prompts to those that did not. We find significant improvement in performance in Psychology and borderline significant improvement in Physics. While the interventions did not raise students' confidence, interviews with students revealed that the prompts helped them process course material and study for exams. This is one of few studies to directly compare the effectiveness of metacognitive prompts across disciplines.