Shailene Woodley eats clay. This fact was established about a month ago in a particularly glorious Into the Gloss interview, and if you have memorized that piece of literature — "it bonds to negative isotopes," etc. — then nothing in this David Letterman interview is new, exactly. But! It does feature Dave asking important questions like "where do you get your clay?" (Colorado!) and "what is the difference between dirt and clay?" What is the difference between dirt and clay? Please advise.