Classical Piece in "Harold & Maude"

at 12 minutes and 25 seconds into the movie, there is a classical piece playing as the mother swims in the pool with Harold pretending to have drowned. What is that piece? It is famous and well known, but it escapes me. Please help. Thanks in advance

Tchaikovsky - Piano Concerto No. 1
http://www.youtube.com/watch?v=_CMo_mLNz38