We can easily measure the duration of the video files typing:
time mplayer movie.avi
Typically we will want to combine multiple video files together, this can be done with the mencoder:
mencoder -ovc copy movie_45.avi movie_46.avi -o result.avi
Not all movies need to be recorded with a video camera, we can also put in between nice animations created for example with the blender. In our movie the wizard writes a letter that is later delivered by a magic bird - this bird is actually an animation.