Double-slit interference is a difficult phenomenon for students to grasp in an introductory physics course. A thorough understanding of constructive and destructive interference is critical, but even with such an understanding, visualizing exactly how an interference pattern is formed by light passing through two slits can still be a challenge for students. Here, we describe a simple demonstration that seems to have improved our students' understanding of this phenomenon. The required materials are readily available in nearly any physics classroom or lab, and the demonstration can easily be constructed in 5 to 10 minutes.