(This video is no longer available for streaming.) Perhaps the greatest mystery about dinosaurs is how did they get so big? "Sue," for example, a Tyrannosaurus rex fossil at Chicago's Field Museum, is fully 20 feet tall and 40 feet long. How she got that big, it turns out, is tied up in yet another mystery: How long did she live? By studying the bones of both extinct dinosaurs and their living reptilian cousins—alligators and lizards—Greg Erickson of Florida State University has been able to answer both questions: Like a reptilian James Dean, T. rex lived fast and died young.