Probably Youtube. The videos from Youtube are streamed, that is, the downloading video bit stream is stored in a buffer file on your computer and your computer starts to display the video as soon as the buffer has x% of the data. If the required data rate for playback of the video exceeds the download rate, the buffer will empty and the video will pause while it builds back to x%. You can hit the pause button on the video player to allow the buffer to fill to a greater value then hit play.
Bob
