Large gender gaps in performance on questions involving projectile motion have been observed at high school and university level, even amongst high-achieving students. This gap is particularly problematic because projectile motion is typically one of the first topics formally taught in physics, and this may give girls an inappropriately negative impression of the subject and their ability to succeed in it. We present a case study describing a particularly problematic conceptual question on projectile motion, and a teaching intervention designed to address the gender gap on this question. The intervention is cheap, quick, and reduces the gender gap in facility on the particular question from around 30% to 10%, while providing excellent learning gains for both male and female students.