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Tear Jerker: Roundhouse
The entire series finale could be argued as one, but especially the speech made by Ivan before "Before You Know It".
Natasha: It's just a TV show!
Ivan: No, it's my family.
Natasha: They're not really your family.
Ivan: You're wrong. We've done a lot of shows together. People have come and gone, but the family stayed together. People have gotten sick, and we all pulled together until they got better, and when they didn't get better, we still pulled together and we were still a family.note A reference to cast member Dominic Lucero, who was diagnosed with lymphoma following Season 1 and appeared in a few Season 3 episodes mid-treatment before succumbing to the disease a month before the taping of the final episode. It wasn't easy, but life went on.
"Before You Know It" itself, and not just because it was the last real song on the show.
As for the people who came and went, besides Dominic, only Crystal Lewis (left after season 1 to pursue her singing career), Jennifer Cihi, Amy Ehlrich and Shawn Munoz (all of which didn't stick around after Season 3) weren't there to send off the show with their comrades.note Lisa Vale was absent in season 3 due to maternity leave, but came back for the final season.
The song "World Be Still" from the Christmas episode. It's a ballad about peace, with surprisingly deep lyrics for a Nickelodeon show.
"The Bridge" as well. Interestingly, it wasn't actually written for the show; it was written and performed by Benny Hester in The Seventies, and released on his debut album.
The ending of the gang violence episode. Sometimes there are problems not even reprising the theme song and rolling the credits can solve - Ivan finds out the hard way, and ends up wandering off confused. Just before the credits show up, we see this text:
There were over 3,000 gang-related fatalities in the U.S. in 1992 alone. After auto accidents, guns are the 2nd leading cause of death among teenagers.
In the U.S. today, there are approximately 5,000 gangs, with over 500,000 members.
Gang violence is no joke.
In a way, the semi-Reality Subtext of the popularity episode, the season 2 premiere, and the first episode with Lisa Vale. Easychair Expressway noticed this about The Eleven O'Clock Number Vale performed for that episode: "Apparently Lisa’s tears at the end of her song are very real, nerves had gotten to her on her first show... but the song was done very well in one take."
The Cinderella episode has the song "I Love You". It's Natasha Pearce's plaintive delivery of the lyrics that seals it. "I heard you were here this evening / no one asked you to stay / I saw you as you were leaving / but there was something that I wanted to say..."