I spent the first two weeks in August as a student at the Summer Science Institute at SLAC.  The topic: "Nature's Greatest Puzzles."  We heard lectures, reports, and had discussion sessions on one puzzle per day for ten days.  I've picked three to talk about in this colloquium: Do neutrinos oscillate?  Where does mass come from?  What is the "dark energy"?  My talk will be a brief introduction to each of these puzzles, plus some of the current "best guesses" about the answers.