Oh, really?

Charlie has been working the craziest words into his vocabulary lately. He's a big fan of "investigate" - as in: "Mommy, we should investigate these trains over here." The other day he said, "What the heck?," which I'm sure I am to blame for (although better than the many alternatives). But yesterday took the cake. He was checking out the zipper on one of the living room pillows, and said, "Daddy, I'm not entirely sure what this is."