We evaluated the effectiveness of the use of an electronic personal response system (or "clickers") during an introductory psychology lecture on perceptual constancy. We graphed and projected student responses to questions during the lecture onto a large-screen display in Microsoft PowerPoint. The distributions of answers corresponded well to results found in the literature. Students rated the lecture as more interactive, interesting, and entertaining. Students in the clicker lecture also performed significantly better on exam questions concerning the lecture compared to another group of students who did not use the clickers. (Contains 2 tables.)