What is this music???



I'm pretty sure the piece is by Mozart, probably from one of the symphonies or operas. I've heard it before, but can't recall the name of the piece.
 
This is the piece:

[ame=http://www.youtube.com/watch?v=Q4SyqVpQgGA]Mozart - Eine kleine Nachtmusik, K.525 - II.Romance : Andante - YouTube[/ame]