A series of images on a strip of 35 mm film, just like you would use in a camera, uncut negatives. It worked like a slide film projector, lit by a bright bulb from behind, projecting the positive color image through a lens onto a surface, either a retractable classroom wall screen or the back of the viewscreen of a viewer. Some filmstrips came with a cassette or phonograph record that would beep when it was time for the operator to advance to the next image. Some viewers could advance the film automatically. Filmstrip topics ranged from culture and government to UFOs and crytopzoology. A student could view a filmstrip at a desktop viewer, similar to a microfilm reader, or a filmstrip could be projected on a screen in a classroom.