Savannah always hated having her diaper changed, but she loved cake. Savannah's mom did not want to feed her cake while changing her diaper though, so she decided to use classical conditioning to make diaper changes less miserable. She started by playing the same song on a CD and giving Savannah a small bite of cake. Once conditioning had occurred, she played the song when changing Savannah's diaper. What is the conditioned stimulus in this case?