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Title: Jessica seeking song...
Review: The song that you're looking for IS on the soundtrack. It's Samantha Ronson's "Built This Way." It's played several times throughout the movie. Once when "Cady" is at the field (when she waves at "Aaron"), another when Aaron sees a picture of a younger Cady on an elephant during the party she throws at her house, and again at the dance, however, the tempo of the song has been slowed down. The uptempo version, so-called, is the version that appears on the soundtrack; perhaps its change in tempo is why you don't recognize it. It's a great song, either way.
