I remember Magnolia seriously affecting the way I looked at movies when I first saw it. Since then, I've found it a lot more melodramatic and junk, but still a great movie.
Honestly, Dazed and Confused completely made me rethink highschool in the last two years of it and has affected how I socialize and spend my time in college to a large degree. It's effect on me has resulted in many good friends and fun times.
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Sex Piano.