Nearly every introductory physics or chemistry course includes live lecture demonstrations, which can range from simple illustrations of a pendulum to elaborate productions with specialized apparatus and highly trained demonstrators. Students and instructors often consider "demos" to be among the highlights of these classes. Yet, in some situations demos may be cumbersome, inaccessible, or otherwise unavailable, and online video demos could offer a convenient alternative. We compared the effectiveness of live demonstrations with online videos under controlled conditions in the first semester of an introductory physics (mechanics) course. Students were randomly assigned to view either a live or video version of two demos. The same instructor presented both versions of the demo using an identical script, keeping the same time on task across both conditions, but with small differences in presentation appropriate to the medium. Compared with the students who saw the live demos, the students who watched the online videos learned more, and their self-reported enjoyment was just as high. We discuss reasons why videos helped students to learn more, including that they are more likely to make correct observations from the video. These results suggest that videos could provide students with an equally effective learning experience when live demos are unavailable. Indeed, even when live demonstrations are available, it may be beneficial to supplement them with online presentations.