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Don McLean's classic tune, American Pie, is the longest song ever to top Billboard's Hot 100, but the single has gone on to make money for the singer/songwriter in ways that he never could've predicted.

The coming-of-age movie, American Pie, earned McLean a nice chunk of change in the late '90s. Although the comedy didn't use the song, producers had to pay McLean to use the name after he had it trademarked.

VIDEO: Don McLean on Being Discovered by a New Generation

The song has proven to be timeless, and with new listeners comes new theories on what the song is really about. The lyric, "The day that music died," is said to be the day that Buddy Holly died in a plane crash, but McLean prefers to keep mum on the song's true meaning.

"There's a whole book that came out about it," said McLean. "I think there's a lot of things people say that I don't even know."

McLean is the subject of a new documentary, Don McLean: American Troubadour, out today.