A movie camera or video camera takes pictures very quickly, usually at 24 or 25 pictures (frames) every second. When a movie projector, a computer, or a television shows the pictures at that rate, it looks like the things shown in the set of pictures are really moving. Sound is either recorded at the same time, or added later. The sounds in a movie usually include the sounds of people talking (which is called dialogue), music (which is called the "soundtrack"), and sound effects, the sounds of activities that are happening in the movie (such as doors opening or guns being fired). In the 20th century the camera used photographic film. The product is still often called a "film" even though there usually is no film.