A how-to on using FFmpeg to copy audio and video streams from one container to another without re-encoding them. Downloading a video from YouTube is pretty easy these days thanks to various browser extensions; and if the downloaded video is in a .mp4 container, I don’t have do anything since just about any video player, including my 1st generation PopcornHour reads them just fine. But some videos on YouTube are stuffed into .flv containers.