When over at a friend's, Jeremy's, house Stacy was introduced to a knew song which instantly connected with her and she loved it. In this phase, the song is the nuetral stimulus, Sarah's reaction to the song is the unconditioned response and her good mood is the unconditioned stimuli.
Phase 1
Wow! This is a great song!
I know right!
Later that week, Stacy had a really tough day, and she decided to listen to the same song that her friend Jeremy had shown her earlier in the week. After listening to it enough, the song began to actually make her feel sad. During this phase, the connection between the song and her emotions is made and the song becomes a controlled stimulus.
1 week later...
Phase 2
*Sob*
Following that day, any time Stacy heard the song Jeremy had introduced to her, she felt an uncontrollable wave of sadness wash over her. This is because the song was now a controlled stimulus that could produce the conditioned response of her feeling upset with the sound of the song alone.