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Tear Jerker: React
"Teens React to Bullying (Amanda Todd)". Dear God, Lia was so upset. (She was in tears even before finding out that Amanda Todd killed herself).
"Teens React to Newtown Shooting". The solemn faces of everyone as they watch President Obama make his speech. Lia comes damned close to tearing up a second time. It even got a sniffle out of Victor, one of the more stoic ones.
"Teens/Elders React To The Election": Rock, after spending the video touting the ideals of Mitt Romney, finally adds a last thought which anybody of any political persuasion can appreciate, which brings him close to tears:
"Kids React to Gay Marriage": Also mostly for being a heartwarming moment like the above, but also a bit sadly was that not all thirteen kids in the episode were for gay marriage. Lucas, the five-year old with Unusual Eyebrows, was the only kid who was against gay marriage; as the video went on his responses were sounding more anti-gay, even though he didn't have a valid reason why gay couples should not marry. (Although, to point this out, he was the youngest of the kids featured in this episode. It's most likely that the reason why he was against gay marriage was simply that he just doesn't know any better yet at his young age, so it's still likely that his opinions may change as he gets older.)
If you get easily upset, don't read the comments for the video. A lot of them are quite depressing and homophobic.
"Teens React to Crying Baby". It's mostly happy tears, although Tom mentions that the song that makes him cry is one that reminds him of his brother overdosing when Tom was 16.