The beginning of this song has a very slow tempo, which slowly builds. It definitely engages the listener, and keeps you listening- waiting for more. There are "rising lines" in the early vocals, going from low to high pitches.  At about 44 seconds into it, there is suddenly a crescendo and an increase in momentum that transitions the song into a more energetic piece.
One thing that I found particularly interesting was to listen to the first 30 seconds and then skip to 2:45 into the song. There is a distinct different, and you can tell which part fits as the beginning and which part fits as the end.   


I chose to examine the structure of the pop song "Oops I did it again" by Britney Spears: